Cleanup for C++20 testing on a Mac M1

[AlisdairM]

This patch supplies additional cleanups that were independently developed testing C++20 on an M1 mac. After rebasing onto the changes already landed in master, the main effect is to adopt an additional library feature macro, in the style of existing feature macros, rather than rely on inline tests for the C++ language version. Also added platform support for the Arm 9 CPU, and fixed up a typo or two.