Fix build with memtest86+ 6.00
memtest86+ 6.00-1 (in testing as of this date) adds and changes its available binaries:
- /boot/memtest86+x32.bin
- /boot/memtest86+x32.efi
- /boot/memtest86+x64.bin
- /boot/memtest86+x64.efi
Future live-build functionality would ideally incorporate these new binaries (x64 for amd64 builds, efi for grub-efi), but in the meantime, this is an MVP update which supports either old memtest86.bin or new memtest86+x32.bin.