scons: Simplify arch enum generation

scons: Simplify arch enum generation

C++ allows a trailing comma after the last item in an enum, so there
is no need for a special case.

Change-Id: I6ead36b4a8562b4a7a5aec88e4f6390182eedf56
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34116
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>