MoltenVK builds compatible with osxcross

Official Khronos MoltenVK releases are incompatible with current osxcross versions due to Xcode automatically using the -fobjc-msgsend-selector-stubs optimization.

The linker used by osxcross doesn't seem to support these yet, even when compiling against Apple LLVM.

See this upstream issue for details:

• https://github.com/KhronosGroup/MoltenVK/issues/1756

This repository simply does it own builds of MoltenVK with these flags added to disable the optimization:

-fno-objc-msgsend-selector-stubs -Wno-unused-command-line-argument

That's the only difference with Khronos' builds.