python,util: Pull enum hh|cc generation out of the MetaEnum class.

Change-Id: Ibfcc2d6916318ffef806f74e57e3f8360489efb6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49452
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
3 files changed