libgo: update configure to current sources

Change-Id: I12766baf02bfdf2233f1c5bde1a270f06b020aa7
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/321076


Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>