Basic support for runtime in shared library. Needs to be built with runtime/build-sharedlibs.sh and activated manually by providing -defaultlib=llvmdc-runtime-shared -defaultlib=tango-cc-tango-shared -defaultlib=tango-gc-basic-shared on llvmdc call at the moment. To support D shared libraries in non-D code, we need to remove the main() from libllvmdc-runtime-shared.