x86: enable "make ARCH=x86"

After unification of the Kconfig files and
introducing K64BIT support in kconfig
it required only trivial changes to enable
"make ARCH=x86".

With this patch you can build for x86_64 in several ways:
1) make ARCH=x86_64
2) make ARCH=x86 K64BIT=y
3) make ARCH=x86 menuconfig
   => select 64-bit

Likewise for i386 with the addition that
i386 is default is you say ARCH=x86.

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>