x86/AMD: unbreak CPU hotplug on AMD systems without RstrFpErrPtrs

x86/AMD: unbreak CPU hotplug on AMD systems without RstrFpErrPtrs

If the feature is not present Xen will try to force X86_BUG_FPU_PTRS
feature at CPU identification time. This is especially noticeable in
PV-shim that usually hotplugs its vCPUs. We either need to restrict this
action for boot CPU only or allow secondary CPUs to modify
forced CPU capabilities at runtime. Choose the former since modifying
forced capabilities out of boot path leaves the system in potentially
inconsistent state.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>