cmd/go: include "gccgo stdlib" in lost build action check

Fixes #32060

Change-Id: I49ab48a9bdd64a7ed777a045e2d92d28a37b5c0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/177600
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/work/action.go

@@ -430,7 +430,7 @@ func (b *Builder) CompileAction(mode, depMode BuildMode, p *load.Package) *Actio
 	// by the install action during (*Builder).installAction.
 	buildAction := a
 	switch buildAction.Mode {
-	case "build", "built-in package":
+	case "build", "built-in package", "gccgo stdlib":
 		// ok
 	case "build-install":
 		buildAction = a.Deps[0]