avr: support ThinLTO

ThinLTO results in a small code size reduction, which is nice
(especially on these very small chips). It also brings us one step
closer to using ThinLTO everywhere.

compileopts/config.go

@@ -200,11 +200,6 @@ func (c *Config) UseThinLTO() bool {
 		// wasm-ld doesn't seem to support ThinLTO yet.
 		return false
 	}
-	if parts[0] == "avr" {
-		// These use external (GNU) linkers which might perhaps support ThinLTO
-		// through a plugin, but it's too much hassle to set up.
-		return false
-	}
 	// Other architectures support ThinLTO.
 	return true
 }