This is necessary to make a relocated GOROOT work correctly.

Fixes #20997

Change-Id: I18624bd2e109721066cd9e4a887a12583ab79f5d
Reviewed-on: https://go-review.googlesource.com/48550
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>