efi/x86: Disallow efi=old_map in mixed mode

Before:

  1f299fad: ("efi/x86: Limit EFI old memory map to SGI UV machines")

enabling the old EFI memory map on mixed mode systems
disabled EFI runtime services altogether.

Given that efi=old_map is a debug feature designed to work around
firmware problems related to EFI runtime services, and disabling
them can be achieved more straightforwardly using 'noefi' or
'efi=noruntime', it makes more sense to ignore efi=old_map on
mixed mode systems.

Currently, we do neither, and try to use the old memory map in
combination with mixed mode routines, which results in crashes,
so let's fix this by making efi=old_map functional on native
systems only.

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