Implement ARM pointer auth stripping

[Gankra]

ARM64 has an optional extension where certain pointers are obfuscated to help mitigate severe memory safety exploits. I have this stubbed out in the soon-to-land ARM64/ARM64_OLD unwinder:

fn ptr_auth_strip(ptr: Pointer) -> Pointer {
    // TODO: attempt to remove ARM pointer authentication obfuscation
    ptr
}

This extension is used on Apple's ARM64 platforms, see: https://bugzilla.mozilla.org/show_bug.cgi?id=1673755

[Gankra]

CC @mstange

[mstange]

Pasting my most recent comment from that bug here:

The easiest fix here would be to pick a fixed mask that works today, and change it once it stops working. 24 bits hash + 40 bits pointer seems to work well. The 24+40 split is also mentioned on https://lwn.net/Articles/718888/ and in various other resources: https://www.google.com/search?q=arm64e+pointer+authentication+40+24+bits

[Gankra]

Relevant libunwind code

[Gankra]

Completely forgot we dug up all this useful info -- so far in my local testing this kinda hacky approach in breakpad seems to work ok? https://searchfox.org/mozilla-central/rev/cc869830d5cb2482a123ed3a63782bfd5dcf74ec/toolkit/crashreporter/google-breakpad/src/processor/stackwalker_arm64.cc#58-79

(currently refining macos arm64 unwinding)

[Gankra]

Minimally implemented in #247