Initial 64bit malloc support when BIT64 is enabled

[RickCarlino]

@crcx I realize you don't use GH or Git for your usual workflow, so we can close this when it is merged.

I look forward to your feedback.

What is your opinion on backward compatibility? This PR removes mem: support from 32-bit builds. It feels strange to have a set of words with different function signatures depending on the bitness of the build. Eg: 32 bit mem: words return double but 64 bit mem: words return cells.

[RickCarlino]

🤔 I guess if people are still building 32 bit kernels or running on true 32 bit hardware, perhaps there might still be interest. 🤷

[crcx]

I think only supporting this on 64-bit builds is acceptable. If I see a demand for supporting it on 32-bit builds, I can reconsider, but I suspect most users are on 64-bit systems now, and even on 32-bit systems, it’s possible to build & use a 64-bit VM and image (I do this on my iPad, under iSH).