[DebugInfo][test][MIPS] Use mtriple in tests

Mips tests are using -march in RUN lines,
this will fail on AIX OS , when we get the mips-ibm-aix triple.

This is caused/exposed recently due to https://reviews.llvm.org/D101194 changed the default getMultiarchTriple in toolchain.

Update the tests to use -mtriple instead to avoid unintended failures.

Reviewed By: atanasyan

Differential Revision: https://reviews.llvm.org/D101863