memtest86plus: disable all hardening

With hardening enabled it reports errors on known-good memory modules
on my Thinkpad X230 (Ivy Bridge). It's the same bug as reported in
https://bugs.launchpad.net/ubuntu/+source/memtest86+/+bug/1071209 but
memtest86+ fails on test NixOS#9 instead of test NixOS#7 (because NixOS#7 in 4.20
became NixOS#9 in 5.01) and with all the addresses multiplied by 2 (I guess
the bug was reported for i686, and I test on x86_64, it was 2012 after
all).

pkgs/tools/misc/memtest86+/default.nix

@@ -42,7 +42,7 @@ stdenv.mkDerivation rec {
 
   NIX_CFLAGS_COMPILE = "-I. -std=gnu90";
 
-  hardeningDisable = [ "fortify" "stackprotector" "pic" ];
+  hardeningDisable = [ "all" ];
 
   buildFlags = "memtest.bin";