Rework -defaultlib/-debuglib; split druntime/Phobos

[dnadlinger]

In preparation for properly supporting shared libraries, it will be necessary to split druntime and Phobos into two separate libraries, instead of also linking in druntime into Phobos statically. I propose calling the new ("slim") Phobos library libphobos2-ldc, to match what DMD does and not mess up versioning for people already trying to use it as a shared library (which is unfortunately done, despite being catastrophically unstable).

We should also take this opportunity to rethink the way -defaultlib/-debuglib work: Currently, specifying the flag multiple times concatenates all the libraries together in a list. However, this makes it impossible for the user to actually override the default (in order to e.g. not link in Phobos altogether or use a different runtime), making the flags much less useful.

Thus, I'd propose to change the flags so that only the last flag counts, with multiple default libraries being specified a single string with delimiters, e.g. -defaultlib="phobos2-ldc;druntime-ldc".