Update README.md. Only low 2GB is usable for LuaJIT x64.

Quoting Mike Pall:

> However, if I utilize any address over 2 GB (any address with the 31st bit
> set) then I get a segfault in 2.1 git head in what appears to be jitted
> code.

No suprise there. I always said the GC-managed memory must be in
the lowest 2 GB of the address space for x64.

[Yes, you can use the full 4 GB in 32 bit mode on x86.]