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.

Change-Id: I68e8249af7e4b3f19809ad03509a884e25d2422f
Reviewed-on: https://go-review.googlesource.com/46153
Reviewed-by: Ian Lance Taylor <iant@golang.org>
2 files changed