x86/setup: Move internal setup_data structures into setup_data.h

Move struct_efi_setup_data in order to unify duplicated definition of
the data structure in a single place. Also silence clang's warnings
about GNU extensions in real-mode code which might occur from the
changed includes.

[ bp: Massage commit message. ]

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240112095000.8952-3-tzimmermann@suse.de