Embed linker directives in ELF and Mach-O object files

[kinke]

Resolves #3245 by adding pragma(lib, <name>) library names to llvm.dependent-libraries for ELF object files.

For Mach-O, embed appropriate linker options for pragma(lib) and support generic pragma(linkerDirective, <flag>, ...) as well.

[kinke]

I left the direct linker cmdline args as-is for now.

For Linux, they are obviously still required as the special section is only recognized by LLD 9+ for now.

For Mac, we only support ld64 IIRC, so the cmdline args might be skippable, relying exclusively on the args embedded in the object files, just like for Windows.

[JohanEngelen]

Nit: all test files need a // REQUIRES: target_X86

[JohanEngelen]

So these directives are not added to metadata, but on the commandline instead? Let's test that?

[kinke]

Nope, they are ignored for Linux. From https://dlang.org/spec/pragma.html#linkerDirective:

The string literal specifies a linker directive to be embedded in the generated object file.

[JohanEngelen]

Ah, now i understand your original PR comment. So on top of COFF support what DMD does, it's only enabled for macOS. Perhaps add a comment about that in the testcase, or simply remove those lines?

[kinke]

Nit: all test files need a // REQUIRES: target_X86

Indeed, I somehow thought generating .ll always works - I'll have to update the Wiki page accordingly too.