cmd/internal/obj: failed TestPCALIGN at read only GOROOT

[mengzhuo]

The linux-mengzhuo-mips64le builder has set goroot as read-only after build completed.

https://build.golang.org/log/97aac799817dbca76dda86e7df6fb044cf1a92fa

git bisect that CL 207117 runs go tool asm -S test.s without clarified the outfile to tmp path.

[andybons]

@cherrymui

[cherrymui]

I already sent CL https://go-review.googlesource.com/c/go/+/225440 for that. Thanks.

[cherrymui]

Fixed by the CL above.