This ought to revert the bad effects of
https://go-review.googlesource.com/#/c/20775/
If you don't pass BOOT_GO_GCFLAGS, you get the
old behavior.

Tweaked to allow multiple space-separated flags in
BOOT_GO_GCFLAGS.

Change-Id: I2a22a04211b4535d1c5a8ec7a8a78cb051161c31
Reviewed-on: https://go-review.googlesource.com/20871
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>