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

[iampi31415]

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.

[photex]

I haven't tried this yet but it can apparently be enabled by signing gdb: https://gist.github.com/mike-myers-tob/9a6013124bad7ff074d3297db2c98247