libgo: update to Go 1.15.5 release

Change-Id: Id7f3bbbbdb7c97944579b0ef0f3c254b4346a041
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/272146
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
8 files changed