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;
 }

testdata/cgo/main.go

@@ -16,4 +16,5 @@ func main() {
 	println("myint size:", int(unsafe.Sizeof(x)))
 	var y C.longlong = -(1 << 40)
 	println("longlong:", y)
+	println("global:", C.global)
 }

testdata/cgo/main.h

@@ -1,2 +1,3 @@
 typedef short myint;
 int add(int a, int b);
+extern int global;

testdata/cgo/out.txt

@@ -3,3 +3,4 @@ add: 8
 myint: 3 5
 myint size: 2
 longlong: -1099511627776
+global: 3