Use GCC atomic intrinsics for PowerPC 32/64 (GCC 4.8+ and clang 3.8+)

Use GCC atomic intrinsics for PowerPC 32/64 (GCC 4.8+ and clang 3.8+)
(Cherry-pick commit 8e62b6f from 'ppc-gcc-atomics' branch.)

* src/atomic_ops/sysdeps/gcc/powerpc.h: Include generic.h and do not
include all_aligned_atomic_load_store.h, test_and_set_t_is_ao_t.h if
GCC 4.8+ or clang 3.8+ unless AO_DISABLE_GCC_ATOMICS.
* src/atomic_ops/sysdeps/gcc/powerpc.h (AO_nop_full, AO_lwsync,
AO_nop_write, AO_nop_read, AO_load_acquire, AO_store_release,
AO_test_and_set, AO_test_and_set_release, AO_test_and_set_full,
AO_compare_and_swap, AO_compare_and_swap_acquire,
AO_compare_and_swap_release, AO_compare_and_swap_full,
AO_fetch_compare_and_swap, AO_fetch_compare_and_swap_acquire,
AO_fetch_compare_and_swap_release, AO_fetch_compare_and_swap_full,
AO_fetch_and_add, AO_fetch_and_add_acquire, AO_fetch_and_add_release,
AO_fetch_and_add_full, AO_T_IS_INT): Do not define if generic.h is
included.