[mkroot] x32 mcm-buildall.sh final patch

[Alain Toussaint]

> The larger question here is "toolchain the kernel builds with and toolchain
> userspace builds with are different".

Yeah, on that count, I need to do a patch for the x86_64 kernel build to include the x32
compatiblities settings. That being said, when I was hacking the prototype, the kernel got built by
the x32 toolchain but I don't know (yet) if a kernel built by the x32 toolchain would see past 4GB.
It did build fine and worked under qemu.

For bare metal, I already had a 64bit kernel with the suitable options on a multiboot usb key.

>  And the OTHER problem is that x86_64-*-native
> now resolves to two toolchains.

That is the bit I was refering to which is why I set the x86_64 arch to x32 on the first patch so
that running ./cross.sh x86_64 ./mkroot dynamic kernel built an x32 kernel and userspace.

Alain