all: add bare-bones Cgo support

testdata/cgo/main.c

@@ -0,0 +1,9 @@
+#include <stdint.h>
+
+int32_t fortytwo() {
+	return 42;
+}
+
+int32_t mul(int32_t a, int32_t b) {
+	return a * b;
+}

testdata/cgo/main.go

@@ -0,0 +1,13 @@
+package main
+
+/*
+#include <stdint.h>
+int32_t fortytwo(void);
+int32_t mul(int32_t a, int32_t b);
+*/
+import "C"
+
+func main() {
+	println("fortytwo:", C.fortytwo())
+	println("mul:", C.mul(int32(3), 5))
+}

testdata/cgo/out.txt

@@ -0,0 +1,2 @@
+fortytwo: 42
+mul: 15