Update darwin_xnu_macho headers from 2017 versions to current Apple open source releases.

[rjmansfield]

Current Darwin xnu/mach-o headers are from 2017 and are missing support for modern Mach-O features used in current macOS/iOS binaries.

Missing things such as:

• Missing load command types that are now standard in modern Apple binaries
• Missing modern CPU types and subtypes

Updating the headers will allow support to be added to bloaty to recognize the new load commands (e.g. #426) and then use the CPU and subtypes in the mach-o processing.

[EricRahm]

Something I'd like to do take make things easier for future maintainers is to document how we update darwin_xnu_macho and perhaps use a script as documentation.

Very hand-wavy, but something like:

mkdir /tmp/xnu && cd /tmp/xnu
git checkout xnu <specific revision>
mkdir darwin_xnu_macho
cp <headers we care about> <our bespoke layout under darwin_xnu_macho>
# patch things up (either with an actual patch file or just sed or ...)
cp -r /tmp/xnu/darwin_xnu_macho <bloaty>/third_party

I'm a little worried that the latest headers have new dependencies which might spiral out into needing to include a much larger set of headers. We should be pragmatic here and be open to some small modifications to remove the extra dependencies if they're not needed for bloaty builds. Don't worry too much about that now for just prototyping things.