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>
1 file changed