main/gcc: Fix D language support on non-x86_64
While some basic functions were working, some C definitions in Druntime were wrong, which led to wrong compilation when trying to bootstrap other packages on x86, ARM and AArch64. Additionally, s390x didn't seem to pick the dependency on the C runtime, and needed an extra dependency on libucontext-dev (not to mention a few fixes to C definitions, like other archs).
- The '402-dlang-stat.patch' content is https://github.com/dlang/druntime/pull/2898 and was merged upstream, as well as its 'stable' counterpart (which the testing/ldc will need);
- The '403-dlang-imbz.patch' content needs to be submitted upstream (to GCC, not dlang);