Add support for Mach-O files and Darwin .a files

[PathogenDavid]

Mach-O is the format macOS uses for executables and dynamic libraries.

It seems Apple no longer hosts documentation on its format (Wikipedia links to this archive). I did find this repo, which helpfully archived the original Apple PDF (Archive) and has a version of it adapted to markdown.

Alternatively we could just use ELFSharp (which contains Mach-O support) for Biohazrd, although this could put us in the same position as #1 where it does way more than we need and requires understanding the Mach-O spec to use properly anyway.

Library archives

Additionally we should support macOS static library archives. Like with everyone else, macOS .a files are a variant of the !<arch> format. Unlike everyone else they use the BSD variant since macOS is BSD-derived. Thankfully this different in format is somewhat easy to identify (there won't be any index file or longname files, and file names begin with #1/ with the real file name being part of the data portion of the file.)

In a BSD archive the equivalent of the index file (__.SYMDEF) is technically optional. macOS might use a variant that is sorted, which is denoted by the file being named __.SYMDEF SORTED. (This was used in the sample library I grabbed from my LLVM build.)

macOS can also apparently have a variant of the symbol table called __.SYMDEF_64, although it's unclear when it appears. (It wasn't there in the sample library.)

The actual object files appear to be Mach-O.

[PathogenDavid]

The header files referenced by the documentation are present in the macOS SDK in /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/.