Merge commit e578d0fd2 from llvm git (by Simon Atanasyan):

[mips] Fix `__mips_isa_rev` macros value for Octeon CPU

This is one of the upstream changes needed for adding support for the
OCTEON+ CPU type, so that we can test Clang builds using the most
commonly available FreeBSD/mips64 reference platform, the Edge Router
Lite.

Requested by: kevans
MFC after: 1 month
X-MFC-With: r353358