[test] Fix tests when run on windows after SVN r369426. NFC.
When running tests on windows, invoking "llc -march=<arch>" will
implicitly use windows as the target os, making these tests misbehave
after this change.
Fix the issue by using more specific -mtriple values instead of plain
-march in these tests.
This should hopefully fix buildbot failures like
http://lab.llvm.org:8011/builders/clang-x64-windows-msvc/builds/9816.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369443 91177308-0d34-0410-b5e6-96231b3b80d8