configurable ABI tags
Downstream of Alpine, we are preparing to distribute musl and glibc variants of an Alpine derivative. Additionally, it is likely desirable to distinguish packages built against macOS, so they are not installed on Linux systems and vice versa.
Accordingly, we extend APK_DEFAULT_ARCH into APK_DEFAULT_ABI_TAG. This allows building apk with -Dabi_tag=foo, in which case, the archs will be glibc-x86_64, or darwin-aarch64.
It may be desirable to allow the user to define APK_DEFAULT_ARCH on her own, for subarchitectures like Adélie's pmmx, but that is not implemented here.