x86/efi: Save and restore FPU context around efi_calls (x86_64)

Do a complete FPU context save/restore around the EFI calls. This required
as runtime EFI firmware may potentially use the FPU.

This change covers only the x86_64 configuration.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Borislav Petkov <bp@suse.de>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>