x86/Kconfig: Add arch attack vector support

ARCH_HAS_CPU_ATTACK_VECTORS should be set for architectures which implement
the new attack-vector based controls for CPU mitigations.  If an arch does
not support attack-vector based controls then an attempt to use them
results in a warning.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250707183316.1349127-4-david.kaplan@amd.com