Documentation/releases/checklist: Add instructions for crossgcc sources

We keep a mirror in case any of the originals disappear, but we also
have to remember to update it.

Change-Id: Ib4be91d1d508d3d5dba7ace1d167d8e528d58b3d
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34508
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

Documentation/releases/checklist.md

@@ -68,6 +68,7 @@ be more frequent than was needed, so we scaled it back to twice a year.
 - [ ] Announce that the release tag is done on IRC
 - [ ] Update release notes with actual commit id, push to repo
 - [ ] Upload release files to web server
+- [ ] Upload crossgcc sources to web server
 - [ ] Update download page to point to files, push to repo
 - [ ] Write and publish blog post with release notes.
 - [ ] Update the topic in the irc channel that the release is done.
@@ -195,6 +196,22 @@ The downloads page is the official place to download the releases from, and it n
 
 Here is an example commit to change it: https://review.coreboot.org/#/c/19515/
 
+## Upload crossgcc sources
+Sometimes the source files for older revisions of
+crossgcc disappear. To deal with that we maintain a mirror at
+https://www.coreboot.org/releases/crossgcc-sources/ where we host the
+sources used by the crossgcc scripts that are part of coreboot releases.
+
+Run
+
+````
+% util/crossgcc/buildgcc -u
+````
+
+This will output the set of URLs that the script uses to download the
+sources. Download them yourself and copy them into the crossgcc-sources
+directory on the server.
+
 ## After the release is complete
 Post the release notes on https://blogs.coreboot.org