Add AArch64 (64-bit ARM) target initial support

* src/Makefile.am (nobase_private_HEADERS): Add gcc/aarch64.h entry.
* src/atomic_ops.h: Include gcc/aarch64.h if __aarch64__.
* src/atomic_ops/sysdeps/gcc/aarch64.h: New file.
Update AUTHORS file
Reformat code of gcc/aarch64.h
Add TODO items to gcc/aarch64.h

* src/atomic_ops/sysdeps/gcc/aarch64.h: Add TODO items.
* src/atomic_ops/sysdeps/gcc/arm.h: Remove FIXME for 64-bit ARM.
Fix AO_nop_full and AO_load_read for AArch64

* src/atomic_ops/sysdeps/gcc/aarch64.h: Place include of
read_ordered.h to the end of file (to be after AO_load definition).
* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_nop_full): Force compiler
barrier for the uni-processor case.
* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_load_acquire): Comment out
and add FIXME (due to read_ordered.h).
* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_test_and_set,
AO_test_and_set_acquire, AO_test_and_set_release,
AO_test_and_set_full): Cast result to AO_TS_VAL_t (since
__atomic_test_and_set returns AO_TS_t value).
Generalize AArch64 add1/sub1 primitives

* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_fetch_and_add1,
AO_fetch_and_add1_acquire, AO_fetch_and_add1_release,
AO_fetch_and_add1_full, AO_fetch_and_sub1, AO_fetch_and_sub1_acquire,
AO_fetch_and_sub1_release, AO_fetch_and_sub1_full): Remove (since
generalized to the same expression).
Merge branch 'master' into add-aarch64-support
Fix AO_load_acquire for AArch64

* src/atomic_ops/sysdeps/gcc/aarch64.h: Add TODO for AO_nop_read;
uncomment AO_load_acquire definition and remove include of
read_ordered.h file (because AO_load_acquire requires "release"
barrier, AO_load_read is defined in generalize-small.h using
AO_nop_read).
Move intrinsic-based primitives from gcc/aarch64.h to new gcc/generic.h
(code refactoring)

* src/Makefile.am (nobase_private_HEADERS): Add generic.h entry.
* src/atomic_ops/sysdeps/gcc/aarch64.h: Move all primitives to
generic.h; include generic.h file.
* src/atomic_ops/sysdeps/gcc/generic.h: New file.
* src/atomic_ops/sysdeps/gcc/generic.h (AO_nop_full): Add comment.
Allow to prefer more generalized primitives in gcc/generic.h

* src/atomic_ops/sysdeps/gcc/generic.h (AO_test_and_set,
AO_test_and_set_acquire, AO_test_and_set_release, AO_test_and_set_full,
AO_fetch_and_add, AO_fetch_and_add_acquire, AO_fetch_and_add_release,
AO_fetch_and_add_full): Do not define if AO_PREFER_GENERALIZED.
* src/atomic_ops/sysdeps/gcc/generic.h (AO_compare_and_swap): Do not
define if AO_GENERALIZE_ASM_BOOL_CAS.
Implement nop_read/write and and/or/xor_acquire/release/full for AArch64

* src/atomic_ops/sysdeps/gcc/generic.h (AO_nop_read, AO_nop_write,
AO_and, AO_and_acquire, AO_and_release, AO_and_full, AO_or,
AO_or_acquire, AO_or_release, AO_or_full, AO_xor, AO_xor_acquire,
AO_xor_release, AO_xor_full): Implement.
* src/atomic_ops/sysdeps/gcc/generic.h (AO_nop_full): Implement using
__atomic_thread_fence instead of __sync_synchronize.
* src/atomic_ops/sysdeps/gcc/generic.h: Add comment about AO_load_full
and AO_store_full; update TODO items.
Implement double-wide load/store and CAS primitives for AArch64

* src/atomic_ops/sysdeps/gcc/aarch64.h: Include standard_ao_double_t.h
(before inclusion of generic.h).
* src/atomic_ops/sysdeps/gcc/generic.h (AO_double_load,
AO_double_load_acquire, AO_double_store, AO_double_store_release,
AO_double_compare_and_swap): Implement (only if
AO_HAVE_DOUBLE_PTR_STORAGE defined).
TODO file: move item about sync-base primitives to aarch64.h and generic.h
Use __atomic GCC built-in to implement generic double-wide CAS

* src/atomic_ops/sysdeps/gcc/generic.h (AO_double_compare_and_swap):
Implement using __atomic_compare_exchange_n(__ATOMIC_RELAXED) instead
of __sync_bool_compare_and_swap.
Fix double_compare_and_swap compile-time error in gcc/generic.h

* src/atomic_ops/sysdeps/gcc/generic.h (AO_double_compare_and_swap):
Fix access to old_val value.
Fix AO_double_t definition for AArch64

* src/atomic_ops/sysdeps/standard_ao_double_t.h (double_ptr_storage):
Define explicitly for AArch64 (use __int128 instead of long long).
Define nop_write as 'dmb st' for AArch64

* src/atomic_ops/sysdeps/gcc/aarch64.h (AO_nop_write): Define via asm
(dmb st) statement unless AO_UNIPROCESSOR (because it seems there is
no equivalent definition using compiler __atomic_thread_fence).
* src/atomic_ops/sysdeps/gcc/generic.h (AO_nop_write): Do not define
if already is.
Move gcc-generic AO_t-wide primitives to generic-small/arithm headers
(code refactoring)

* src/atomic_ops/sysdeps/gcc/generic-small.h: New file.
* src/atomic_ops/sysdeps/gcc/generic-arithm.h: Likewise.
* src/atomic_ops/sysdeps/gcc/generic.h: Include generic-small.h file.
* src/atomic_ops/sysdeps/gcc/generic.h (AO_load, AO_load_acquire,
AO_store, AO_store_release, AO_fetch_compare_and_swap,
AO_compare_and_swap): Move to generic-small.h file.
* src/atomic_ops/sysdeps/gcc/generic.h: Include generic-arithm.h file
unless AO_PREFER_GENERALIZED.
* src/atomic_ops/sysdeps/gcc/generic.h (AO_fetch_and_add,
AO_fetch_and_add_acquire, AO_fetch_and_add_release,
AO_fetch_and_add_full, AO_and, AO_and_acquire, AO_and_release,
AO_and_full, AO_or, AO_or_acquire, AO_or_release, AO_or_full, AO_xor,
AO_xor_acquire, AO_xor_release, AO_xor_full): Move to generic-arithm.h
file.
* src/Makefile.am (nobase_private_HEADERS): Add gcc/generic-arithm.h,
gcc/generic-small.h entries.
Implement char/short/int-wide primitives using GCC built-in atomic/sync

* src/Makefile.am (EXTRA_DIST): Add gcc/generic-arithm.template,
gcc/generic-small.template entries.
* src/Makefile.am (nobase_private_HEADERS): Move gcc/generic-arithm.h,
atomic_ops/sysdeps/gcc/generic-small.h to BUILT_SOURCES.
* src/Makefile.am (atomic_ops/sysdeps/gcc/generic-arithm.h,
atomic_ops/sysdeps/gcc/generic-small.h): New rules (to generate
AO_char/short/int_X primitives included from gcc/generic.h file.
* src/atomic_ops/sysdeps/gcc/generic-arithm.template: New template
file (based on code moved from generic-arithm.h).
* src/atomic_ops/sysdeps/gcc/generic-small.template: New template file
(based on code moved from generic-small.h).
* src/atomic_ops/sysdeps/gcc/generic-arithm.h: Regenerate.
* src/atomic_ops/sysdeps/gcc/generic-small.h: Likewise.
Minimize gcc/generic-arithm template by factoring out barriers
(code refactoring)

* src/Makefile.am (atomic_ops/sysdeps/gcc/generic-arithm.h): Iterate
over XBAR/XGCCBAR (in addition to XSIZE/XCTYPE).
* src/atomic_ops/sysdeps/gcc/generic-arithm.template: Parametrize all
primitives with XBAR (replacing AO none/acquire/release/full barriers)
and with XGCCBAR (replacing GCC atomic RELAXED/ACQUIRE/RELEASE/SEQ_CST
barriers, respectively).
* src/atomic_ops/sysdeps/gcc/generic-arithm.h: Regenerate.