edk2/OVMF: Support build on macOS

In order to use OVMF firmware with e.g. qemu on macOS, these packages
needed to be made macOS ready. This meant choosing the clang build in
this case, because it is the only one working on macOS.

Unfortunately, just using clang on all platforms doesn't work because
there are hardcoded assumptions in the edk2 build system.