cmd/go: fix build failure in -x test
cmd/go: fix build failure in -x test

CL 84735 strengthened the -x test to make sure commands succeed,
using set -e, but the gcc flag tests can fail. Change them to say || true.

Fixes #23337.

Change-Id: I01e4017cb36ceb147b56935c2636de52ce7bdfdb
Reviewed-on: https://go-review.googlesource.com/86239
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

josharian/go,Cofyc/go,cloudflare/go,codestation/go,codestation/go,nawawi/go,nawawi/go,akutz/go,CAFxX/go,cloudflare/go,codestation/go,cloudflare/go,akutz/go,golang/go,codestation/go,akutz/go,akutz/go,nawawi/go,Cofyc/go,CAFxX/go,akutz/go,cloudflare/go,josharian/go,golang/go,CAFxX/go,josharian/go,cloudflare/go,codestation/go,Cofyc/go,Cofyc/go,cloudflare/go,Cofyc/go,Cofyc/go,nawawi/go,CAFxX/go,golang/go,golang/go,Cofyc/go,codestation/go,golang/go,josharian/go,Cofyc/go,nawawi/go,nawawi/go,codestation/go,akutz/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,golang/go,golang/go,josharian/go,CAFxX/go,akutz/go,CAFxX/go,akutz/go,josharian/go,codestation/go,josharian/go,cloudflare/go,nawawi/go,josharian/go,christopher-henderson/Go