x86/fpu: Provide a proper function for ex_handler_fprestore()

To make upcoming changes for support of dynamically enabled features
simpler, provide a proper function for the exception handler which removes
exposure of FPU internals.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011540.053515012@linutronix.de