kernel/x86: add ability to set GS segment base from userland

Needed for Wine.

Co-authored-by: Jérôme Duval <jerome.duval@gmail.com>

Change-Id: I13f6a5802fce04fd2ebb4cc01ecd2f12c90830db
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4839
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>
Reviewed-by: X512 <danger_mail@list.ru>
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>