[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.

llvm-svn: 369443