Fix PROT_EXEC restrictions on Apple Silicon

[Techcable]

Currently, on Apple Silicon chips, calling mmap(PROT_WRITE | PROT_EXEC) will fail due to restrictions on marking memory executable & writable at the same time.

See apple developer documentation on this new restriction: https://developer.apple.com/documentation/apple-silicon/porting-just-in-time-compilers-to-apple-silicon

This workaround simply stops requesting PROT_EXEC entirely on Apple Silicon. This is not a complete solution, because it does not allow clients to use seamlessly garbage collection to manage their executable memory.

However, it does get basic support working. IT works fine as long as you don't use the MPS to directly allocate executable memory.

See also issue #75 (which is not fully resolved by this)

Note that any "proper" solution would still require signficant work to interact with the new Apple APIs for JIT. This means that it is almost certainly going to require some effort on the part of the client, and possibly new APIs on the part of the MPS.

After this commit, the example scheme interpreter builds and runs correctly on my Apple M1 :)

Note the ./configure script still doesn't work correctly on Apple Silicon, but I figure that's an issue for another PR 😉