go/build: don't alter InstallSuffix for default compile options

Fixes #16378.

Change-Id: I99a064f1afec78fb63cb3719061d20be0f21d45d
Reviewed-on: https://go-review.googlesource.com/24930
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testshared/shared_test.go

@@ -43,7 +43,7 @@ func run(t *testing.T, msg string, args ...string) {
 }
 
 // goCmd invokes the go tool with the installsuffix set up by TestMain. It calls
-// t.Errorf if the command fails.
+// t.Fatalf if the command fails.
 func goCmd(t *testing.T, args ...string) {
 	newargs := []string{args[0], "-installsuffix=" + suffix}
 	if testing.Verbose() {
@@ -63,7 +63,7 @@ func goCmd(t *testing.T, args ...string) {
 	}
 	if err != nil {
 		if t != nil {
-			t.Errorf("executing %s failed %v:\n%s", strings.Join(c.Args, " "), err, output)
+			t.Fatalf("executing %s failed %v:\n%s", strings.Join(c.Args, " "), err, output)
 		} else {
 			log.Fatalf("executing %s failed %v:\n%s", strings.Join(c.Args, " "), err, output)
 		}

src/cmd/go/build.go

@@ -431,10 +431,13 @@ func buildModeInit() {
 			buildAsmflags = append(buildAsmflags, codegenArg)
 			buildGcflags = append(buildGcflags, codegenArg)
 		}
-		if buildContext.InstallSuffix != "" {
-			buildContext.InstallSuffix += "_"
+		// Don't alter InstallSuffix when modifying default codegen args.
+		if buildBuildmode != "default" || buildLinkshared {
+			if buildContext.InstallSuffix != "" {
+				buildContext.InstallSuffix += "_"
+			}
+			buildContext.InstallSuffix += codegenArg[1:]
 		}
-		buildContext.InstallSuffix += codegenArg[1:]
 	}
 }