Code refactoring of gcc/powerpc.h to avoid code duplication

Code refactoring of gcc/powerpc.h to avoid code duplication

* src/atomic_ops/sysdeps/gcc/powerpc.h (AO_PPC_LD, AO_PPC_LxARX,
AO_PPC_CMPx, AO_PPC_STxCXd, AO_PPC_LOAD_CLOBBER): New internal macros
(undefined at the end of the file).
* src/atomic_ops/sysdeps/gcc/powerpc.h (AO_T_IS_INT): Move definition
upper (to have single check of __ppc64__).
* src/atomic_ops/sysdeps/gcc/powerpc.h (AO_load_acquire): Use AO_PPC_LD,
AO_PPC_LOAD_CLOBBER (and eliminate code duplication).
* src/atomic_ops/sysdeps/gcc/powerpc.h (AO_test_and_set,
AO_compare_and_swap, AO_fetch_compare_and_swap, AO_fetch_and_add): Use
AO_PPC_LxARX, AO_PPC_CMPx, AO_PPC_STxCXd (and eliminate code
duplication).