cmd/go: compile packages imported by runtime with -+

cmd/go: compile packages imported by runtime with -+

Packages imported by the runtime should follow the same memory
allocation rules as the runtime package.

Change-Id: Id3fadb05605acbd6ac61d7fe6068a2add8bb2308
Reviewed-on: https://go-review.googlesource.com/105995
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>