windows: use ThinLTO

Switch Windows builds to use ThinLTO. This gets us closer to using
ThinLTO everywhere.

compileopts/config.go

@@ -192,11 +192,6 @@ func (c *Config) UseThinLTO() bool {
 		// through a plugin, but it's too much hassle to set up.
 		return false
 	}
-	if len(parts) >= 2 && parts[2] == "windows" {
-		// Linker error (undefined runtime.trackedGlobalsBitmap) when linking
-		// for Windows. Disable it for now until that's figured out and fixed.
-		return false
-	}
 	// Other architectures support ThinLTO.
 	return true
 }