Document that MacOS aarch64 is not available for debugging with GDB.

Link: https://rustc-dev-guide.rust-lang.org/debugging-support-in-rustc.html

If we try debugging in mac os natively we get:

> Don't know how to run.  Try "help target".

because it is not supported by GDB yet. I think we should make this clear, incl any workaround that people may use.