llvm MIPS target: add missed source file

Otherwise, linking llvm binaries with this target enabled (which is not
the default) will fail with a number of undefined symbol errors:

ld: error: undefined symbol: llvm::initializeMipsMulMulBugFixPass(llvm::PassRegistry&)
ld: error: undefined symbol: llvm::createMipsMulMulBugPass()

MFC after: 3 days

(cherry picked from commit 4b1174e27a30f9a627586c5d45e10ba7e801d3bd)