cmd/go: fix clang option handling

On OpenBSD -current, clang is available/installed as 'cc'. This means that
the existing clang check fails and the clang related flags are not enabled.
Fix this by enabling the clang flags if the compiler claims to support them.

Change-Id: I84d124d2409a10f87002c6cbfdb69b4c9a55981a
Reviewed-on: https://go-review.googlesource.com/60750Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/work/build.go

@@ -3173,10 +3173,12 @@ func (b *Builder) ccompilerCmd(envvar, defcmd, objdir string) []string {
 		}
 	}
 
-	if strings.Contains(a[0], "clang") {
-		// disable ASCII art in clang errors, if possible
+	// disable ASCII art in clang errors, if possible
+	if b.gccSupportsFlag("-fno-caret-diagnostics") {
 		a = append(a, "-fno-caret-diagnostics")
-		// clang is too smart about command-line arguments
+	}
+	// clang is too smart about command-line arguments
+	if b.gccSupportsFlag("-Qunused-arguments") {
 		a = append(a, "-Qunused-arguments")
 	}