cmd/compile: fix wrong package path for unsafe.Pointer

It's not a predeclared type, but a type defined in "unsafe" package.

Fixes #44830

Change-Id: If39815b1070059b608be8231dfac9b7f3307cb15
Reviewed-on: https://go-review.googlesource.com/c/go/+/313349
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/reflectdata/reflect.go

@@ -595,7 +595,7 @@ func typePkg(t *types.Type) *types.Pkg {
 			}
 		}
 	}
-	if tsym != nil && t != types.Types[t.Kind()] && t != types.ErrorType {
+	if tsym != nil && tsym.Pkg != types.BuiltinPkg {
 		return tsym.Pkg
 	}
 	return nil

test/fixedbugs/issue44830.go

@@ -0,0 +1,19 @@
+// run
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"reflect"
+	"unsafe"
+)
+
+func main() {
+	t := reflect.TypeOf(unsafe.Pointer(nil))
+	if pkgPath := t.PkgPath(); pkgPath != "unsafe" {
+		panic("unexpected t.PkgPath(): " + pkgPath)
+	}
+}