Fix powerpc detection on AIX (gcc)

GCC/powerpc-ibm-aix does not define any of __powerpc__, __ppc__,
__PPC__, __powerpc64__, __ppc64__; but it does define _ARCH_PPC (and
_POWER).

* src/atomic_ops.h [_ARCH_PPC]: Include gcc/powerpc.h.
ivmai/libatomic_ops@b6bfe95
Author: Tobias Leich <email@froggs.de>
Date:   Fri Nov 18 01:05:42 2016 +0300

3rdparty/libatomic_ops/src/atomic_ops.h

@@ -266,7 +266,8 @@
 #   include "atomic_ops/sysdeps/gcc/nios2.h"
 # endif /* __nios2__ */
 # if defined(__powerpc__) || defined(__ppc__) || defined(__PPC__) \
-     || defined(__powerpc64__) || defined(__ppc64__)
+     || defined(__powerpc64__) || defined(__ppc64__) \
+     || defined(_ARCH_PPC)
 #   include "atomic_ops/sysdeps/gcc/powerpc.h"
 # endif /* __powerpc__ */
 # if defined(__aarch64__)