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/86239Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/work/exec.go

@@ -1799,7 +1799,7 @@ func (b *Builder) gccSupportsFlag(compiler []string, flag string) bool {
 	// GCC and clang.
 	cmdArgs := str.StringList(compiler, flag, "-c", "-x", "c", "-")
 	if cfg.BuildN || cfg.BuildX {
-		b.Showcmd(b.WorkDir, "%s", joinUnambiguously(cmdArgs))
+		b.Showcmd(b.WorkDir, "%s || true", joinUnambiguously(cmdArgs))
 		if cfg.BuildN {
 			return false
 		}