ERL: set -march=octeon+ for use with external toolchain

Upstream GCC and devel/mips64-gcc use "octeon+" as the CPU setting for
the Octeon processor in the EdgeRouter Lite. As of r312899 the base
system GCC 4.2.1 accepts octeon+ as an alias for the Octeon support
added in r208737 for the same CPU.

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