add -shared-druntime flag, add new meaning for -defaultlib=

This implements phobos2 being split into libphobos2.a and libdruntime.a. With -shared-druntime flag, libdruntimeso.so is linked to instead. -defaultlib= with no library specified causes neither phobos2 nor druntime to be linked it (necessary to build libdruntime.so).

Relies on https://github.com/D-Programming-Language/phobos/pull/1223

and https://github.com/D-Programming-Language/druntime/pull/462