compiler: fix int casting to use the source signedness

Previously, when casting an integer to a bigger integer, the destination
signedness was used. This is problematic when casting a negative int16
to uint32, for example, because it would cause zero-extension.

src/runtime/print.go

@@ -39,7 +39,7 @@ func printuint16(n uint16) {
 	printuint32(uint32(n))
 }
 
-func printint16(n uint16) {
+func printint16(n int16) {
 	printint32(int32(n))
 }