Add -d:nimNoGetEntropy to allow running on macOS earlier than 10.12

[iffy]

Related to #17370 but not the same. This change enables me to compile on a newer macOS a program that runs on an older macOS.

This does not restore the ability to compile on older macOS.

[Araq]

Why nimNoGetEntropy? Why not e.g. nimOldOsX?

[iffy]

Why nimNoGetEntropy? Why not e.g. nimOldOsX?

I was trying to match nimNoGetRandom (which is adjacent to this) and be specific to this feature rather than possibly conflicting with a more general thing. I could imagine there being nimOldOsX now and nimSlightlyNewerOldOsX later :) Ideally, I'd rather make the code just work for old and new, but that would require doing a runtime check rather than a compile-time check and I don't know how to do that.

[iffy]

Wait, I've got some idea how I could do it with dynlib. Let me try again.

[iffy]

Okay #20466 is the better fix.