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

arch/x86/Kconfig

@@ -133,7 +133,7 @@ config X86
 	select ARCH_SUPPORTS_AUTOFDO_CLANG
 	select ARCH_SUPPORTS_PROPELLER_CLANG    if X86_64
 	select ARCH_USE_BUILTIN_BSWAP
-	select ARCH_USE_CMPXCHG_LOCKREF		if X86_CX8
+	select ARCH_USE_CMPXCHG_LOCKREF
 	select ARCH_USE_MEMTEST
 	select ARCH_USE_QUEUED_RWLOCKS
 	select ARCH_USE_QUEUED_SPINLOCKS

arch/x86/Kconfig.cpu

@@ -269,7 +269,6 @@ config X86_HAVE_PAE
 
 config X86_CX8
 	def_bool y
-	depends on X86_HAVE_PAE || M586TSC || M586MMX || MK6 || MK7 || MGEODEGX1 || MGEODE_LX
 
 # this should be set for all -march=.. options where the compiler
 # generates cmov.

arch/x86/Kconfig.cpufeatures

@@ -42,7 +42,6 @@ config X86_REQUIRED_FEATURE_NOPL
 
 config X86_REQUIRED_FEATURE_CX8
 	def_bool y
-	depends on X86_CX8
 
 # this should be set for all -march=.. options where the compiler
 # generates cmov.