x86/kconfig: Move 64-bit only arch Kconfig selects to 'config X86_64'

These are easier to read when they come next to the X86_64 config.

Note that all remaining 'if X86_64' config options in the generic
section are in principle suitable for activation on 32-bit, but have
not been ported yet.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>