scons: Remove the AddLocalOption workaround.

scons: Remove the AddLocalOption workaround.

The "append" option of the Help() scons method can be used to avoid
clobbering the built in and local option help.

This has the nice side effect of making it easier to add options in
other files since you now only need the built in AddOption provided by
scons itself, not the custom AddLocalOption version.

Change-Id: Ifa566087797d578df0c90f8f4fca70c8152fbf63
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32115
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>