Use GCC atomic intrinsics for ARM (gcc 4.8+ and clang 3.5+)

Use GCC atomic intrinsics for ARM (gcc 4.8+ and clang 3.5+)

* src/atomic_ops/sysdeps/gcc/arm.h (AO_GCC_ATOMIC_TEST_AND_SET): Define
if gcc 4.8+ or clang 3.5+ or AO_PREFER_BUILTIN_ATOMICS unless
AO_DISABLE_GCC_ATOMICS.
* src/atomic_ops/sysdeps/gcc/arm.h (AO_nop_write, AO_store,
AO_char_store, AO_short_store): Do not define (in assembly code) if
AO_PREFER_BUILTIN_ATOMICS.
* src/atomic_ops/sysdeps/gcc/arm.h (AO_SKIPATOMIC_store,
AO_SKIPATOMIC_store_release, AO_SKIPATOMIC_char_store,
AO_SKIPATOMIC_char_store_release, AO_SKIPATOMIC_short_store,
AO_SKIPATOMIC_short_store_release, AO_SKIPATOMIC_int_store,
AO_SKIPATOMIC_int_store_release): Define if AO_BROKEN_TASKSWITCH_CLREX.
* src/atomic_ops/sysdeps/gcc/arm.h (AO_nop_full, AO_test_and_set,
AO_fetch_and_add, AO_fetch_and_add1, AO_fetch_and_sub1, AO_and, AO_or,
AO_xor, AO_char_fetch_and_add, AO_short_fetch_and_add,
AO_compare_and_swap, AO_fetch_compare_and_swap, AO_double_load,
AO_double_store, AO_double_compare_and_swap, AO_test_and_set_full): Do
not define (in assembly code) if AO_GCC_ATOMIC_TEST_AND_SET.
* src/atomic_ops/sysdeps/gcc/arm.h: Do not include atomic_store.h,
all_atomic_only_load.h, char_atomic_store.h, short_atomic_store.h,
all_aligned_atomic_load_store.h if AO_GCC_ATOMIC_TEST_AND_SET.
* src/atomic_ops/sysdeps/gcc/arm.h: Include generic.h (at the end of
the file) if AO_GCC_ATOMIC_TEST_AND_SET.
* src/atomic_ops/sysdeps/gcc/generic-small.h: Regenerate.
* src/atomic_ops/sysdeps/gcc/generic-small.template (AO_XSIZE_store):
Do not define if AO_SKIPATOMIC_XSIZE_store.
* src/atomic_ops/sysdeps/gcc/generic-small.template
(AO_XSIZE_store_release): Do not define if
AO_SKIPATOMIC_XSIZE_store_release.