lang/gcc/11, lang/gcc/15: add dlang support

ports@,

I'd like to put back dlang for amd64, aarch64 and sparc64.

gcc15 needs gdc from gcc9 so, I had fixed and prebuild bootstraps for both
gcc-11 and gcc-15.

Here a new one which I've asked to be mirrored:
- https://kirill.korins.ky/pub/distfiles/adastrap-aarch64-11.2.0-1.tar.xz
- https://kirill.korins.ky/pub/distfiles/adastrap-amd64-11.2.0-7.tar.xz
- https://kirill.korins.ky/pub/distfiles/bootstrap-aarch64-15.2.0-0.tar.xz
- https://kirill.korins.ky/pub/distfiles/bootstrap-amd64-15.2.0-0.tar.xz
- https://kirill.korins.ky/pub/distfiles/bootstrap-sparc64-15.2.0-0.tar.xz

On each machine it was tested as:

gcc-11 -> gcc-15 (via gcc-11 bootstrap) -> gcc-15 (via gcc-15 bootstrap)

Diff is large (~60k) so I had attached it as gzip.

Anyone for tests? Ok?

--
wbr, Kirill