libobjc2: New package

[qmfrederik]

libobjc2 is a modern Objective C runtime.

The v2.2 release of libobjc2 supports building on msys2 (at least mingw64 and ucrt64, via gnustep/libobjc2#254 and gnustep/libobjc2#267); and the clang patches which enable this (llvm/llvm-project#77797, llvm/llvm-project#77385, llvm/llvm-project#77255) have been backported to msys (#19767).

This PR adds libobjc2 to msys2.

Closes #11942
Closes #8287

[lazka]

The installed files look wrong:

  +/ucrt64/lib/objc.dll
  +/ucrt64/lib/objc.lib

This should be:

  +/ucrt64/bin/objc.dll
  +/ucrt64/lib/objc.dll.a

[qmfrederik]

@lazka Thanks, I created a patch to fix the bin vs lib mismatch and hope to get it upstreamed. The .dll.a vs .lib difference seems to stem from using clang instead of gcc (libobjc2 supports clang only). I couldn't find a CMake variable which would influence this behavior, so I renamed the file in the package phase.

[qmfrederik]

ok - that patch landed upstream just now: gnustep/libobjc2#275