cgo: supports c/go callback funcs

cl/_testrt/struct/out.ll

@@ -2,6 +2,7 @@
 source_filename = "main"
 
 %main.Foo = type { i32, i1 }
+%"github.com/goplus/llgo/internal/runtime.eface" = type { ptr, ptr }
 
 @"github.com/goplus/llgo/internal/runtime.cgoAlwaysFalse" = external global i1, align 1
 @main.format = global [10 x i8] zeroinitializer, align 1
@@ -35,6 +36,15 @@ _llgo_0:
   ret void
 }
 
+define ptr @main._Cgo_ptr(ptr %0) {
+_llgo_0:
+  ret ptr %0
+}
+
+declare void @runtime.cgoUse(%"github.com/goplus/llgo/internal/runtime.eface")
+
+declare void @runtime.cgoCheckResult(%"github.com/goplus/llgo/internal/runtime.eface")
+
 define void @main.init() {
 _llgo_0:
   %0 = load i1, ptr @"main.init$guard", align 1