x86: improve NOP use for AMD CPUs

x86: improve NOP use for AMD CPUs

For Fam10 and later AMD recommends using the "long" NOP forms. Re-write
the present Intel code into switch() statements and add AMD logic.

Default to "long" forms (which all 64-bit CPUs are supposed to
recognize), overriding to the K8 flavor on those few (older) CPUs.

This at the same time brings us in line again in this regard with
current Linux.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>