build: test linux/mips64 and linux/mips64le with buildall.bash

build: test linux/mips64 and linux/mips64le with buildall.bash

Both mips64 architectures share the same runtime/rt0 file, so
we have to hardcode them in buildall.bash.

Ideally we should have cmd/dist report all supported platforms,
see #12270.

Change-Id: I08ce35cfe0a831af5e1e8255b305efd38386fa52
Reviewed-on: https://go-review.googlesource.com/18687
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>