Call powerpc-darwin external tools with -arch ppc.

With this patch we call external tools for powerpc-darwin with "-arch ppc"
instead of "-arch powerpc", so as to be compatible with the cctools assembler
and ld64 linker.

Patch by Stephen Drake!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216687 91177308-0d34-0410-b5e6-96231b3b80d8