cgo: do not rely on stdint.h to be available

testdata/cgo/main.go

@@ -1,20 +1,20 @@
 package main
 
 /*
-#include <stdint.h>
-int32_t fortytwo(void);
-int32_t mul(int32_t a, int32_t b);
-typedef int32_t myint;
+int fortytwo(void);
+typedef short myint;
 int add(int a, int b);
 */
 import "C"
 
+import "unsafe"
+
 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))
+	println("myint size:", int(unsafe.Sizeof(x)))
 	var y C.longlong = -(1 << 40)
 	println("longlong:", y)
 }