x86: copy x86_64 specific Kconfig symbols to Kconfig.i386

No functional changes.
A prepatory step towards full unification.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>