avr: use a garbage collector

This might sound crazy, but I think it's better to enable the GC by
default to avoid surprises. It costs 1130 bytes of flash and 16 bytes of
RAM (plus heap overhead) so it's not exactly free, but if needed it can
easily be disabled with `-gc=leaking`. On the Uno (32kB flash, 2kB RAM)
that's not massive, on the DigiSpark (8kB flash, 0.5kB RAM) that may be
too much depending on the application.

src/runtime/gc_conservative.go

@@ -318,7 +318,7 @@ func markRoots(start, end uintptr) {
 		}
 	}
 
-	for addr := start; addr != end; addr += unsafe.Sizeof(addr) {
+	for addr := start; addr != end; addr += unsafe.Alignof(addr) {
 		root := *(*uintptr)(unsafe.Pointer(addr))
 		markRoot(addr, root)
 	}