In r208737 jmallett@ added support for the "mips64r2" architecture
and "octeon" CPU, and the saa/saad instructions.
Upstream binutils also added the "octeon+" CPU, and the saa/saad
instructions are only available in octeon+, not octeon. Since our
base system tool chain already accepts saa/saad with -march=octeon,
just allow octeon+ as an alias.
This allows the use of octeon+ in kernel config files, for use with both
external tool chain and in-tree GCC/binutils.
PR: 216516
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
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