Factor common userland and baremetal C functions

This allows add.c to run unmodified on both!

For that to work, use int main on baremetal, and pass the return value to
the final exit.

baremetal/interactive/exit1.c

@@ -1,6 +1,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-void main(void) {
+int main(void) {
 	exit(1);
 }