libgo: use gc's arch names as the default GOARCHs on MIPS

libgo: use gc's arch names as the default GOARCHs on MIPS

    This means that the gc tools and gofrontend agree on the architecture names
    for the 3 MIPS ABIs which should allow a gofrontend compiler to build go.

    Reviewed-on: https://go-review.googlesource.com/46153

From-SVN: r249476