As described in https://github.com/torch/torch7/issues/1035, it seems that LuaJIT provides only "a limited range of 47 bits for the legacy lightuserdata data type". Therefore, lua-torch-torch7 can only be built and run on systems that use virtual addresses with no more than 47 bits. Today many arm64 systems, including the buildds, use 48-bit virtual addresses.
For what it's worth, I have seen lua-torch-torch7 built successfully on an arm64 system with 42-bit virtual addresses (and 64K pages) so there does not seem to be any other obstacle to making it work on arm64.
