Build both osx and osx-arm64 versions in a directory called osx.

Build both osx and osx-arm64 versions in a directory called osx.

When DCP-o-matic makes its universal binary it assumes that the
path to the arm64 library can be found by replacing x86_64 with
arm64 in the path.  This fails if we build the x86_64 version in
build_osx and the arm64 version in build_osx-arm64.