x86: disable static NOPLs on 32 bits

On 32-bit, at least the generic nops are fairly reasonable, but the
default nops for 64-bit really look pretty sad, and the P6 nops really do
look better.

So I would suggest perhaps moving the static P6 nop selection into the
CONFIG_X86_64 thing.

The alternative is to just get rid of that static nop selection, and just
have two cases: 32-bit and 64-bit, and just pick obviously safe cases for
them.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>