This refactoring required to make possible BUILD_BC_LIBS option work
together with options like BUILD_SINGLE_LIB and to build Phobos as
LLVM bytecode too.
- libdruntime-rt-ldc can be built as LLVM bytecode library. This feature
was commented out because of currently irrelevant reason.
- Removed irrelevant modifications of native code library content inside
BUILD_BC_LIBS logic.
- Decoupled BUILD_BC_LIBS logic from basic runtime building logic.