Re-commit "Add support for the new mips-mti-linux toolchain."

r249137 added support for the new mips-mti-linux toolchain. However,
the new tests of that commit, broke some buildbots because they didn't use
the correct regular expressions to capture the filename of Clang & LLD.

This commit re-applies the changes of r249137 and fixes the tests in
r249137 in order to match the filenames of the Clang and LLD executable.

llvm-svn: 249294