Check for powerpc arch is wrong

[barracuda156]

There are two problems with the current check for powerpc arch:

1. On Darwin uname -m returns Power Macintosh, and that breaks the script; what could be used instead is uname -p, however that determines cpu family, not the target arch. The correct way to find target arch would be from the triple.
2. Existing code sets powerpc64 for powerpc:

# Normalize TARGET_ARCH to powerpc
ifneq ($(filter ppc powerpc,$(TARGET_ARCH)),)
  TARGET_ARCH := powerpc
endif

# Normalize TARGET_ARCH to powerpc64
ifneq ($(filter ppc% powerpc%,$(TARGET_ARCH)),)
  TARGET_ARCH := powerpc64
endif

# Normalize TARGET_ARCH to powerpc64le
ifneq ($(filter ppc%le powerpc%le,$(TARGET_ARCH)),)
  TARGET_ARCH := powerpc64le
endif

It should be ifneq ($(filter ppc64 powerpc64,$(TARGET_ARCH)),), otherwise it will always be powerpc64, even on 32-bit.