add octeon+ as an alias for octeon in GCC & binutils

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