go/ssa: use sync.Once instead of sync.atomic

This adds a missing memory barrier at the end of Package.Build.

Change-Id: Ife35d5ad5a48ba121f35656fef682863d4f2aef6
Reviewed-on: https://go-review.googlesource.com/14761
Reviewed-by: Robert Griesemer <gri@golang.org>
2 files changed