[PPC] Remove Darwin support from POWER backend.

This patch removes uses of the Darwin ABI for PowerPC related test cases. This
is the first step in removing Darwin support from the POWER backend.

clang/test/CodeGen/darwin-ppc-varargs.c  was deleted because it was a darwin/ppc
specific test case.

All other tests were updated to remove the darwin/ppc specific invocation.

Phabricator Review: https://reviews.llvm.org/D50989.

llvm-svn: 340770