efi/arm64: libstub: run image in place if randomized by the loader

If the loader has already placed the EFI kernel image randomly in
physical memory, and indicates having done so by installing the 'fixed
placement' protocol onto the image handle, don't bother randomizing the
placement again in the EFI stub.

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