libgo: update to 1.15.6 release

Change-Id: I427d58fad3ce23e68348089e8992eb696d458548
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/276153
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
10 files changed