libgo: update to Go1.15.2 release

Change-Id: I7c50226413f75b689ff55356a25f78f61320d09a
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/256618
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
33 files changed