cgo: add support for C.int, c.uint, etc

testdata/cgo/main.c

@@ -4,6 +4,10 @@ int32_t fortytwo() {
 	return 42;
 }
 
+int add(int a, int b) {
+	return a + b;
+}
+
 int32_t mul(int32_t a, int32_t b) {
 	return a * b;
 }

testdata/cgo/main.go

@@ -5,12 +5,16 @@ package main
 int32_t fortytwo(void);
 int32_t mul(int32_t a, int32_t b);
 typedef int32_t myint;
+int add(int a, int b);
 */
 import "C"
 
 func main() {
 	println("fortytwo:", C.fortytwo())
 	println("mul:", C.mul(C.int32_t(3), 5))
+	println("add:", C.add(C.int(3), 5))
 	var x C.myint = 3
 	println("myint:", x, C.myint(5))
+	var y C.longlong = -(1 << 40)
+	println("longlong:", y)
 }

testdata/cgo/out.txt

@@ -1,3 +1,5 @@
 fortytwo: 42
 mul: 15
+add: 8
 myint: 3 5
+longlong: -1099511627776