arch-x86: Convert segment indices to fit the style guide.

Capitalize only their first letter, and use a namespace to namespace
them instead of a SEGMENT_REG_ prefix.

Change-Id: I69778c8d052ad6cc0ffd9e74dd1c643e9d28048d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49756
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
16 files changed