x86/fpu/xstate: Move remaining xfeature helpers to core

Now that everything is mopped up, move all the helpers and prototypes into
the core header. They are not required by the outside.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211014230739.514095101@linutronix.de