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/18687Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/buildall.bash

@@ -33,7 +33,7 @@ if [ "$pattern" = "" ]; then
 fi
 
 # put linux, nacl first in the target list to get all the architectures up front.
-targets="$((ls runtime | sed -n 's/^rt0_\(.*\)_\(.*\)\.s/\1-\2/p'; echo linux-386-387 linux-arm-arm5) | sort | egrep -v android-arm | egrep "$pattern" | egrep 'linux|nacl')
+targets="$((ls runtime | sed -n 's/^rt0_\(.*\)_\(.*\)\.s/\1-\2/p'; echo linux-386-387 linux-arm-arm5) | sort | sed -e 's|linux-mips64x|linux-mips64 linux-mips64le|' | egrep -v android-arm | egrep "$pattern" | egrep 'linux|nacl')
 $(ls runtime | sed -n 's/^rt0_\(.*\)_\(.*\)\.s/\1-\2/p' | egrep -v 'android-arm|darwin-arm' | egrep "$pattern" | egrep -v 'linux|nacl')"
 
 ./make.bash || exit 1