Fix the MIPS baremetal build

Summary:
Currently sys/cachectl.h is used unconditionally on MIPS although it is
only available on Linux and will fail the build when targeting baremetal

Reviewers: petarj

Reviewed By: petarj

Subscribers: sdardis, krytarowski

Differential Revision: https://reviews.llvm.org/D40659

llvm-svn: 319455