tests: Fix makefile sed rule for list_atomic

* tests/Makefile.am (test_atomic_include.h, list_atomic.c): Replace
all occurrences of 'XX' (instead of only the first one) on every line.
Support ARMv8 target (gcc/arm)

* src/atomic_ops/sysdeps/gcc/arm.h (__ARM_ARCH_8A__): Detect new macro
(treated same as __ARM_ARCH_7A__).
* src/atomic_ops/sysdeps/gcc/arm.h (AO_ARM_HAVE_SWP): Do not define
for ARMv8 (since SWP{B} obsoleted); add comment.
* src/atomic_ops/sysdeps/gcc/arm.h (AO_compare_and_swap): Add TODO
item (for deprecated IT block containing wide Thumb instruction).
Tests: workaround GCC 4.4.3 warning reported for list_atomic.c 'val'

* tests/list_atomic.template (XSIZE_list_atomicXX): Use "static"
modifier for "val" local variable to initialize it (cannot use direct
assignment to zero because "val" type could be a SIMD vector type if
XCTYPE is AO_double_t) to workaround GCC v4.4.3
"'val' is used uninitialized in this function" warning reported for
AO_load.
Fix ARMv7 LDREXD/STREXD double-wide operand specification (GCC/Clang)

* src/atomic_ops/sysdeps/gcc/arm.h (AO_double_load, AO_double_store,
AO_double_compare_and_swap): Specify that LDREXD and STREXD use 2
adjacent registers (thus preventing Clang3.3 from register allocation
failures leading to "registers may not be the same" or
"even register required" GAS errors).