x86/asm: Rewrite sync_core() to use IRET-to-self

Aside from being excessively slow, CPUID is problematic: Linux runs
on a handful of CPUs that don't have CPUID.  Use IRET-to-self
instead.  IRET-to-self works everywhere, so it makes testing easy.

For reference, On my laptop, IRET-to-self is ~110ns,
CPUID(eax=1, ecx=0) is ~83ns on native and very very slow under KVM,
and MOV-to-CR2 is ~42ns.

While we're at it: sync_core() serves a very specific purpose.
Document it.

Signed-off-by: Andy Lutomirski <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: One Thousand Gnomes <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Matthew Whitehead <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Henrique de Moraes Holschuh <[email protected]>
Cc: Andrew Cooper <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: xen-devel <[email protected]>
Link: http://lkml.kernel.org/r/5c79f0225f68bc8c40335612bf624511abb78941.1481307769.git.luto@kernel.org
Signed-off-by: Thomas Gleixner <[email protected]>

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