D lang: cross-compile with Druntime + Phobos2

Currently, the available example, it can be seen that non-native build only `-betterC` mode is supported. (This is not on purpose - I'm not anti-GC).

Based on an experiment using meson (by @denizzzka). I ported to zig-build (with ABS), however the version used did not yet support cross-compilation in D, which had already been corrected previously.

- [ldc2-druntime - zig build/gist](https://gist.github.com/kassane/45f91db17771bb3a5b920c9bb885c52b)

To guarantee this possibility, you would need to obtain access to the source code of Druntime and Phobos2, both standalone.
In the case of fortran-runtime (current release already available) is builded using `zig c++` and included on:
https://github.com/kassane/anotherBuildStep/blob/a1f40f90f17fa6d2681f9d46cf8dbc37d4f1ea89/toolchains/flang.zig#L176-L184