libgo: don't use \? in grep pattern

It's not supported by Solaris grep.  Just use * instead; it matches
more but it shouldn't matter.

Fixes https://gcc.gnu.org/PR91764

Change-Id: Id009211ff476c15359394522d2eb37a36aed26c0
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/195238
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>