cmd/dist: restrict parallel tests to NumCPU on mips64x

mips64 builder and one machine of the mips64le builder has small amount
of memory. Since CL 18199, they have been running slowly, as more
processes were launched in running 'test' directory, and a lot of swap
were used. This CL brings all.bash from 5h back to 3h on Loongson 2E
with 512 MB memory.

Change-Id: I4a22e239a542a99ba5986753205d8cd1f4b3d3c6
Reviewed-on: https://go-review.googlesource.com/18483Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/util.go

@@ -461,7 +461,7 @@ func main() {
 		}
 	}
 
-	if gohostarch == "arm" {
+	if gohostarch == "arm" || gohostarch == "mips64" || gohostarch == "mips64le" {
 		maxbg = min(maxbg, runtime.NumCPU())
 	}
 	bginit()