[driver][mips] Hardcode triple name in case of CodeSourcery toolchain. NFC

CodeSourcery toolchain is a standalone toolchain which always uses
the same triple name in its paths. It is independent from target
triple used by the driver.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270067 91177308-0d34-0410-b5e6-96231b3b80d8