misc: Add .mailmap file

misc: Add .mailmap file

This file helps map committers' ids to a canonical name and email. This
is useful for tracking the same committer over time.

I've done my best to map the ids manually. I've tried to choose current
institutions/emails for the people that I personally know, but I am sure
to have missed many. Feel free to correct your own!

Change-Id: I17d57368a2ecb056025a6dabef37485ec5ce6aa9
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29672
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>