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