efi: libstub: Give efi_main() asmlinkage qualification

To stop the bots from sending sparse warnings to me and the list about
efi_main() not having a prototype, decorate it with asmlinkage so that
it is clear that it is called from assembly, and therefore needs to
remain external, even if it is never declared in a header file.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>