HACKING: update GCC repo checkout instructions from SVN to Git

Use the read-only GCC Git repository to get the GCC source code.

Change-Id: Iebb0cbddf097e74ffc8e60819e59386e6c928457
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/237898
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/HACKING b/HACKING
index fb48bb4..6ab5371 100644
@@ -23,9 +23,9 @@
 (or use the mirror on github: git clone http://github.com/golang/gofrontend)
 That gives you the frontend only.  Now you need a copy of the GCC
-source code.  See http://gcc.gnu.org/svn.html for details, or simply:
+source code.  See http://gcc.gnu.org/git.html for details, or simply:
-svn checkout svn://gcc.gnu.org/svn/gcc/trunk
+git clone git://gcc.gnu.org/git/gcc
 The GCC source code will have a copy of the Go frontend, but because
 you want to change the Go frontend, you will need to replace that copy