scons: Update some scons bug report URLs.

scons: Update some scons bug report URLs.

It appears that scons bugs are not on tigris.org any more and are now
on github, although fortunately old bugs seem to have been ported over
and have the same numbering.

This CL updates URLs which were in comments in the gem5 source,
specifically in scons scripts, to point to the corresponding github
version.

I also checked to see if these bugs were still open, or if we could
remove our workarounds for them.

1. 2356 is still open, and has been fairly recently assigned.
2. 2611 is marked as fixed. We might be able to implement the
   workaround in its last comment from August of 2019.
3. 2811 has been marked fixed, and as best I can tell the fix first
   appeared in around version 3.0 of scons. If/when that is our
   minimum version, we can remove the workaround in
   site_scons/site_tools/default.py. That is mostly fixing an annoying
   spurious rebuild by scons which does not affect correctness, so even
   if we remove that workaround we shouldn't break earlier versions,
   although it would be obnoxious for people that are affected by it
   and best avoided.

Change-Id: I0d74820f399044c6f80148bf3022d07d7bf6f4e5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32114
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>