x86: add X86_64 dependency to x86_64 specific symbols in Kconfig.x86_64

To ease unification of Kconfig.i386 and Kconfig.x86_64
add X86_64 dependencies to all x86_64 specific symbols.

This patch introduce no functional changes but is one step
towards unification. This smaller step is used to ease
review of the patch set.

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>