util: Improve the usage text of the m5 utility.

Make sure there *is* help text for every command, alphabetize the
commands, explain all the arguments and what the command as a whole
does.

Change-Id: Icf74f8c638545c8658c99169dce728f7b1127806
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27231
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
1 file changed