main/gcc: Add support for the D programming language
GDC, the GCC-based D compiler, is part of GCC since GCC9. Upstream support for Musl was added progressively (D relies heavily on libc), and some key components were missing as of GCC 9.2.0 release (2019-08-12). A stream of patches, from which the massive patch included in this PR derives, was submitted to the gcc-patches mailing list on the 2019-08-21. This patch could be removed after/if GCC 9.3.0 is released.
This was originally submitted as https://github.com/alpinelinux/aports/pull/12006
Disabled on ppc64le (the compiler build, but not the runtime): https://github.com/D-Programming-GDC/gcc/blob/a65d5ae024a1d42bc8a0a6590e1945305f7281e3/libphobos/configure.tgt