broken ldc2 ebuilds

[Alessandro-Barbieri]

Today they removed llvm:10 from ::gentoo and all the ldc2 ebuilds are now broken

[necrose99]

https://github.com/ldc-developers/ldc/releases/tag/v1.29.0
Platform support
Supports LLVM 6.0 - 14.0. (new)
Basic compiler support for Newlib targets, i.e., triples like arm-none-newlibeabi. (ldc-developers/ldc#3946) (new)