[PowerPC] Fix ppc64-elf-abi.ll test case on Darwin
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@214179 91177308-0d34-0410-b5e6-96231b3b80d8

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