Revert: r211588 - [mips] Use __clear_cache builtin instead of cacheflush() in Unix...

Revert: r211588 - [mips] Use __clear_cache builtin instead of cacheflush() in Unix Memory::InvalidateInstructionCache()

Buildbot reports a test failure on the llvm-mips-linux builder and blames r211588.
Although it doesn't appear in the blamelist, it seems it could also be r211587
(because it's committed to compiler-rt?) since they were tested together.

Reverting the most likely suspect (r211588) to confirm one way or the other.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211594 91177308-0d34-0410-b5e6-96231b3b80d8