efi/libstub: Drop __pure getters for EFI stub options

The practice of using __pure getter functions to access global
variables in the EFI stub dates back to the time when we had to
carefully prevent GOT entries from being emitted, because we
could not rely on the toolchain to do this for us.

Today, we use the hidden visibility pragma for all EFI stub source
files, which now all live in the same subdirectory, and we apply a
sanity check on the objects, so we can get rid of these getter
functions and simply refer to global data objects directly.

So switch over the remaining boolean variables carrying options set
on the kernel command line.

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