avr: add support for recover()

You can see that it works with the following command:

    tinygo run -target=simavr ./testdata/recover.go

This also gets the following tests to pass again:

    go test -run=Build -target=simavr -v

Adding support for AVR was a bit more compliated because it's also
necessary to save and restore the Y register.

src/runtime/arch_tinygoriscv.go

@@ -5,6 +5,8 @@ package runtime
 
 import "device/riscv"
 
+const deferExtraRegs = 0
+
 func getCurrentStackPointer() uintptr {
 	return uintptr(stacksave())
 }