sim: Deprecate M5_AT_* constants.
sim: Deprecate M5_AT_* constants.

These are formatted as #defines, but they are actually enum constants.
If they were actually macros, they should use a GEM5_ prefix. Put them
in the gem5 namespace to remove the need for that prefix, and an auxv
namespace to remove the need for the AT prefix.

Since we now have a gem5::auxv namespace, move the AuxVector template
into it and deprecate the old, global namespace version.

Also, one of the constants was M5_BASE_PLATFORM, which was inconsistent
with the rest of the constants which were M5_AT_... This was not
actually because the constant in any header files was different, it was
just incorrectly transcribed and should have been M5_AT_BASE_PLATFORM.
Since we're changing names anyway, replace this with
gem5::auxv::BasePlatform.

Change-Id: Ie0dc34a683b113a4ed4bc76054eb822676f6e802
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45242
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Reviewed-by: Daniel Carvalho <91779fc734d358d9575ec4910de29bfcbcaf1b22@yahoo.com.br>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5