loader: support global variables in CGo (#173)

Global variables (like functions) must be declared in the import "C" preamble and can then be used from Go.

testdata/cgo/main.c

@@ -1,5 +1,7 @@
 #include "main.h"
 
+int global = 3;
+
 int fortytwo() {
 	return 42;
 }