x86/fpu: Add basic helpers for dynamically enabled features

To allow building up the infrastructure required to support dynamically
enabled FPU features, add:

 - XFEATURES_MASK_DYNAMIC

   This constant will hold xfeatures which can be dynamically enabled.

 - fpu_state_size_dynamic()

   A static branch for 64-bit and a simple 'return false' for 32-bit.

   This helper allows to add dynamic-feature-specific changes to common
   code which is shared between 32-bit and 64-bit without #ifdeffery.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211021225527.10184-8-chang.seok.bae@intel.com