ld64.lld doesn't respect -macos_version_min

Over the years, Apple's ld64 has accepted multiple ways of specifying the target OS version. The current flag is `-platform_version`, but it has previously been `-macos_version_min` and `-macosx_version_min`.

Apple's ld64 accepts all three, but lld only accepts the latest one. This is a major issue when using gcc, which prefers `-macos_version_min`.

The end result is that ld64.lld **is not** a drop in replacement for ld64, as any usage results in:

```
ld64.lld: error: must specify -platform_version
```

Start by tracing ld64.lld's option parsing and the handling of the required target OS version. Verify how -platform_version, -macos_version_min, and -macosx_version_min are currently recognized. The work is done when the older aliases are accepted and a GCC-generated invocation no longer fails with the missing -platform_version error.

Written by the indexing model from the issue text.