efistub/x86: Add fallback for SMBIOS record lookup

efistub/x86: Add fallback for SMBIOS record lookup

Some Apple EFI firmwares do not provide the SMBIOS Protocol,
causing efi_get_smbios_record() to fail. This prevents retrieval of
system information such as product name, which is needed by
apple_set_os() to enable the integrated GPU on dual-graphics Intel
MacBooks.

Add a fallback that directly parses the SMBIOS entry point table when
the protocol is unavailable.

Signed-off-by: Francesco Pompo <francescopompo2@gmail.com>
[ardb: cosmetic tweaks]
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>