[compiler-rt] Implement __clear_cache() on OpenBSD/mips64

Summary:
Make __clear_cache() invoke the platform's cache flush function
on OpenBSD/mips64.

Reviewers: krytarowski

Reviewed By: krytarowski

Subscribers: sdardis, dberris, arichardson, krytarowski, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D42332

llvm-svn: 323039