efi/libstub: zboot: Mark zboot EFI application as NX compatible

Now that the zboot loader will invoke the EFI memory attributes protocol
to remap the decompressed code and rodata as read-only/executable, we
can set the PE/COFF header flag that indicates to the firmware that the
application does not rely on writable memory being executable at the
same time.

Cc: <stable@vger.kernel.org> # v6.2+
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>