Merge commit bf996f761 from llvm git (by Simon Atanasyan):

[mips] Write `AFL_EXT_OCTEONP` flag to the `.MIPS.abiflags` section

Differential Revision: https://reviews.llvm.org/D69851

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