Support TBD v5 in Mach-O linker

Notably, v5 has a more compact JSON form that we can use for `lib/libc/darwin/libSystem.tbd`.

For comparison:

```
❯ llvm-readtapi lib/libc/darwin/libSystem.tbd --filetype=tbd-v4 | wc -c
337439
❯ llvm-readtapi lib/libc/darwin/libSystem.tbd --filetype=tbd-v5 | wc -c
425458
❯ llvm-readtapi lib/libc/darwin/libSystem.tbd --filetype=tbd-v5 -compact | wc -c
258486
```