It was only really necessary for ensuring that package runtime should
be treated as safe even without a "safe" marker, but mkbuiltin.go now
compiles it with -u.

Change-Id: Ifbcc62436ce40ab732ece667141afd82c1d3b64b
Reviewed-on: https://go-review.googlesource.com/19625Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>