mips: XFAIL llvm-cov test

XFAIL llvm-cov.test for MIPS until big-endian issues are fixed for llvm-cov.
The test does pass on MIPS little-endian.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195966 91177308-0d34-0410-b5e6-96231b3b80d8