Moved compiler list out into dlang-compilers.eclass and added functionality to check which compilers are stable. This can be used to only give "stable" packages only USE-flags for compilers which are also stable (as marked in dlang-compilers.eclass). Some traces of D1 were also removed from dlang.eclass. I plan on removing them from eselect dlang as well.
Now that dlang.eclass can filter unstable compiler USE-flags, I stabilized the first D package: dub-0.9.21
Bumped version of dmd and added the magic "-*" arch keyword to mean "no other architectures than amd64 and x86 are supported by dmd binary releases. x32 is probably coming though.