efi/x86: Drop 'systab' member from struct efi

The systab member in struct efi has outlived its usefulness, now that
we have better ways to access the only piece of information we are
interested in after init, which is the EFI runtime services table
address. So instead of instantiating a doctored copy at early boot
with lots of mangled values, and switching the pointer when switching
into virtual mode, let's grab the values we need directly, and get
rid of the systab pointer entirely.

Tested-by: Tony Luck <tony.luck@intel.com> # arch/ia64
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>