Unify the CPU features vectors between i386 and x86-64

Unify the handling of the CPU features vectors between i386 and x86-64.
This also adopts the collapsing of features which are required at
compile-time into constant tests from x86-64 to i386.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>