libgo: handle go1.10+ correctly in match.sh

match.sh was not correctly handling build constraints for Go versions
that have a two-digit suffix, like "go1.10".

The same issue will arise with Go 1.100, but that is a long ways off.

Change-Id: I0d41d60aad800d681a12ede68fbe0763289ec268
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/260077
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Than McIntosh <thanm@google.com>