[ELF][MIPS] Temporarily mark failed MIPS tests as XFAIL.

microMIPS 64-bit is unsupported by LLVM starting from r335057. But such 
code can be generated by GCC. Mark failed test cases as XFAIL while
decide to drop microMIPS 64-bit support from LLD too or use binary
inputs for the test.                               

llvm-svn: 335059