Fix building on MacOS X on PowerPC.

Fix building on MacOS X on PowerPC.

bup failed to build on one of my machines, an older iMac; make
died ~40 lines in with "gcc-4.0: Invalid arch name : Power".

On PPC machines, uname -m returns the helpfully descriptive
"Power Macintosh", which gcc doesn't recognize. Some googling
revealed e.g.
http://www.opensource.apple.com/source/ld64/ld64-95.2.12/unit-tests/include/common.makefile
where they use $(shell arch) to get the necessary info.

With that little change, bup built on ppc and i386 machines for
me, and passed all tests.