Use GCC atomic intrinsics for Hexagon (clang 3.9+)

Use GCC atomic intrinsics for Hexagon (clang 3.9+)

* src/atomic_ops/sysdeps/gcc/hexagon.h [AO_CLANG_PREREQ(3, 9)
&& !AO_DISABLE_GCC_ATOMICS] (AO_GCC_FORCE_HAVE_CAS,
AO_GCC_HAVE_double_SYNC_CAS): Define.
* src/atomic_ops/sysdeps/gcc/hexagon.h [AO_CLANG_PREREQ(3, 9)
&& !AO_DISABLE_GCC_ATOMICS]: Include standard_ao_double_t.h, generic.h.
* src/atomic_ops/sysdeps/gcc/hexagon.h [AO_CLANG_PREREQ(3, 9)
&& !AO_DISABLE_GCC_ATOMICS]: Do not include test_and_set_t_is_ao_t.h,
all_aligned_atomic_load_store.h.
* src/atomic_ops/sysdeps/gcc/hexagon.h [AO_CLANG_PREREQ(3, 9)
&& !AO_DISABLE_GCC_ATOMICS]: Do not define AO_nop_full,
AO_fetch_and_add, AO_test_and_set, AO_compare_and_swap,
AO_fetch_compare_and_swap, AO_T_IS_INT.