Use GCC atomic intrinsics for x86 and x64 (gcc 4.8+ and clang 3.4+)

Use GCC atomic intrinsics for x86 and x64 (gcc 4.8+ and clang 3.4+)

* src/atomic_ops/sysdeps/gcc/generic.h (AO_GCC_HAVE_double_SYNC_CAS):
Do not define if AO_SKIPATOMIC_double_compare_and_swap_ANY.
* src/atomic_ops/sysdeps/gcc/x86.h (AO_GCC_ATOMIC_TEST_AND_SET): Define
if gcc4.8+ (but not Intel compiler) or clang3.4+ (if not
AO_DISABLE_GCC_ATOMICS).
* src/atomic_ops/sysdeps/gcc/x86.h [AO_GCC_ATOMIC_TEST_AND_SET]: Do not
include all_aligned_atomic_load_store.h, test_and_set_t_is_char.h,
ordered_except_wr.h.
* src/atomic_ops/sysdeps/gcc/x86.h (AO_nop_full, AO_fetch_and_add_full,
AO_char_fetch_and_add_full, AO_short_fetch_and_add_full, AO_and_full,
AO_or_full, AO_xor_full, AO_test_and_set_full,
AO_compare_and_swap_full) [AO_GCC_ATOMIC_TEST_AND_SET]: Do not define.
* src/atomic_ops/sysdeps/gcc/x86.h: Include standard_ao_double_t.h if
AO_PREFER_BUILTIN_ATOMICS, or AO_GCC_ATOMIC_TEST_AND_SET and not
clang/x86.
* src/atomic_ops/sysdeps/gcc/x86.h (AO_double_load_acquire,
AO_double_store_release, AO_SKIPATOMIC_double_compare_and_swap_ANY):
Define if AO_GCC_ATOMIC_TEST_AND_SET and clang/x86 (unless
AO_PREFER_BUILTIN_ATOMICS).
* src/atomic_ops/sysdeps/gcc/x86.h
(AO_compare_double_and_swap_double_full) [x86]: Define also for
clang/x86 if AO_GCC_ATOMIC_TEST_AND_SET (unless
AO_PREFER_BUILTIN_ATOMICS).
* src/atomic_ops/sysdeps/gcc/x86.h
(AO_compare_double_and_swap_double_full) [x86_64]: Remove TODO item;
check __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 to detect presence of
cmpxchg16b.
* src/atomic_ops/sysdeps/gcc/x86.h: Include generic.h if
AO_GCC_ATOMIC_TEST_AND_SET.
* src/atomic_ops/sysdeps/standard_ao_double_t.h (double_ptr_storage):
Use unsigned __int128 instead of __m128 for GCC x86_64 atomic
intrinsics.