PowerPC: Do not use llc -march in tests.

`llc -march` is problematic because it only switches the target
architecture, but leaves the operating system unchanged. This
occasionally leads to indeterministic tests because the OS from
LLVM_DEFAULT_TARGET_TRIPLE is used.

However we can simply always use `llc -mtriple` instead. This changes
all the tests to do this to avoid people using -march when they copy and
paste parts of tests.

This patch:
- Removes -march if the .ll file already has a matching `target triple`
  directive or -mtriple argument.
- In all other cases changes -march=ppc32/-march=ppc64 to
  -mtriple=ppc32--/-mtriple=ppc64--

See also the discussion in https://reviews.llvm.org/D35287

llvm-svn: 309754