This adds the PPC architecture variants for Mach-O libbacktrace.

With this (as for X86 and Arm) when dsymutil is run on the binary
we get a basic usable backtrace.

Testsuite results on powerpc-apple-darwin9 are the same as for X86:
 * btest fails (TBC why)
 * dwarf5 tests fail because dsymutil does not handle that so far.

libbacktrace/ChangeLog:

	* macho.c (MACH_O_CPU_TYPE_PPC): New.
	(MACH_O_CPU_TYPE_PPC64): New.
	Add compile-tests for powerpc to the Mach-O variants.

Close #55