Fails on iOS because CC_FOR_TARGET points to clangwrap.sh in the
original GOROOT. We could fix that but it doesn't seem worth it.

Fails on Android with "exec format error". I'm not sure why but I
doubt it is interesting.

Fails on Plan 9 because the original GOROOT is being preserved in some
unknown way. This is issue #21016.

Updates #21016

Change-Id: I4e7115d734fc7bf21e5a2ba18fb6ad0bfa31c735
Reviewed-on: https://go-review.googlesource.com/48650
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>