cmd/cgo: use strings.TrimSuffix

cmd/cgo: use strings.TrimSuffix

Change-Id: I97b5592c678c350fd77069d7c40a98864733707a
Reviewed-on: https://go-review.googlesource.com/c/go/+/435946
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>