libgo: update to Go 1.15.4 release

Change-Id: I79ce5e20afecf729642d011c4c2afaa56cdf0508
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/268177
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
16 files changed