qemu-barrier.h tests if macro __powerpc__ is defined, however, the
preprocessor on PowerPC Mac OS X defines only __POWERPC__, not
__powerpc__. Resolve by testing instead for qemu-provided _ARCH_PPC.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>