x86/cpu: Make CONFIG_X86_CX8 unconditional

x86/cpu: Make CONFIG_X86_CX8 unconditional

I.e. from now on it's a boot time requirement for the CPU to support it.

( This was preceded by the removal of all non-CX8 platform build
  options, so it should have no functional effects in theory. )

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ahmed S . Darwish <darwi@linutronix.de>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: H . Peter Anvin <hpa@zytor.com>
Cc: John Ogness <john.ogness@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250425084216.3913608-14-mingo@kernel.org