zig ld: ability to link tbds on macOS

[kubkon]

Currently, zig ld can successfully link object files .o, static libraries .a, and (limited to no re-exports) dynamic libraries .dylib. However, starting with macOS 11 aka BigSur, no actual system dynamic libraries are available for browsing on the filesystem (well, they are available somewhere in the dyld's global cache). Instead, the linker is supposed to use the so-called text-based definition files .tbds which are compact representation of the symbol tables found within the actual dynamic library and its re-exports. zig ld cannot link tbds yet and they are required if anyone wants to build stage2 with Homebrew's Zig and/or LLVM since LLVM is built with system libxml2 (which is only available as a tbd on recent macOS).

[kubkon]

Hmm, I might try adding some prelim support for this for 0.8.0. If I made it before we cut the release, we'd have a fully functional Zig binary available via Homebrew.

[mikdusan]

some background that may help...

• docs from latest tapi-1100.0.11 library: here
• when tbd appeared:

version	SDK	note
4	11.0	Big Sur
3	10.13	High Sierra, ~4 years ago
2	10.12	Sierra, ~ 5 years ago
1	10.10	Yosemite, ~7 years ago

• and this indication that tapi-12.0.23 is coming:

[mikdusan]

Looking distantly into the future, perhaps .tbd files could be used by zig linkers for other formats to cross-link; it feels like a better solution than hacking stub .so files.

[kubkon]

Since there's quite a bit of work involved in adding this in including a yaml parser, I'm moving this into 0.9.0 milestone.