x86/efistub: Enable SMBIOS protocol handling for x86

The smbios.c source file is not currently included in the x86 build, and
before we can do so, it needs some tweaks to build correctly in
combination with the EFI mixed mode support.

Reviewed-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>