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_cortexm.go

@@ -12,6 +12,8 @@ const GOARCH = "arm"
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 32
 
+const deferExtraRegs = 0
+
 // Align on word boundary.
 func align(ptr uintptr) uintptr {
 	return (ptr + 3) &^ 3