Define double-wide ordered loads/stores for x86

* src/Makefile.am (atomic_ops/sysdeps/loadstore/ordered_loads_only.h,
atomic_ops/sysdeps/loadstore/ordered_stores_only.h): Generate
primitives for AO_double_t as well (from the template).
* src/atomic_ops/sysdeps/gcc/x86.h: Move include of ordered_except_wr.h
(together with the comment) to the end of the file to be after
inclusion of double_atomic_load_store.h (because the former uses the
primitives defined the latter).
* src/atomic_ops/sysdeps/msftc/x86.h: Likewise.
* src/atomic_ops/sysdeps/sunc/x86.h: Likewise.
* src/atomic_ops/sysdeps/loadstore/ordered_loads_only.h: Regenerate.
* src/atomic_ops/sysdeps/loadstore/ordered_stores_only.h: Likewise.
Add TODO items about missing double-wide operations for some targets

* src/atomic_ops/sysdeps/gcc/mips.h: Add TODO items (about double-wide
operations like load/store and CAS).
* src/atomic_ops/sysdeps/gcc/powerpc.h: Likewise.
* src/atomic_ops/sysdeps/gcc/s390.h: Likewise.
* src/atomic_ops/sysdeps/msftc/arm.h: Likewise.
Cast away volatile on dereference in CAS-based generalization primitives

* src/atomic_ops/generalize-arithm.template
(AO_XSIZE_fetch_and_add_full, AO_XSIZE_fetch_and_add_acquire,
AO_XSIZE_fetch_and_add_release, AO_XSIZE_fetch_and_add,
AO_XSIZE_and_full, AO_XSIZE_or_full, AO_XSIZE_xor_full): Cast away the
volatile (when reading old value before CAS) for architectures like
IA64 (where volatile adds barrier semantics).
* src/atomic_ops/generalize-small.template (AO_XSIZE_load_read,
AO_XSIZE_load_full, AO_XSIZE_load_acquire, AO_XSIZE_load): Likewise.
* src/atomic_ops/generalize-arithm.h: Regenerate.
* src/atomic_ops/generalize-small.h: Likewise.
Add AO_store generalization based on CAS

* src/atomic_ops/generalize-small.template (AO_XSIZE_store_write,
AO_XSIZE_store, AO_XSIZE_store_release, AO_XSIZE_store_full): Add
generalization template primitive based on CAS.
* src/atomic_ops/generalize-small.h: Regenerate.
Add AO_double_store pthread-based implementation and tests

* src/atomic_ops/sysdeps/generic_pthread.h (AO_double_store_full): New
primitive.
* tests/test_atomic_include.template (test_atomicXX): Add test of
AO_double_storeXX.