[PowerPC] Fix ppc64-elf-abi.ll test case on Darwin

Use full -mtriple instead of just -march to ensure Linux ABI
(ELFv1 or ELFv2) is selected.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214179 91177308-0d34-0410-b5e6-96231b3b80d8