[llvm] [flang-rt] Use --as-needed for linking flang-rt libraries. (PR #130856)

[Slava Zakharin via llvm-commits]

vzakhari wrote:

> > It works for the shared libraries build, I checked.
> 
> I think thats what `set_target_properties(${tgtname} PROPERTIES LINKER_LANGUAGE C)` already does, which just does not add `-lc++`, respectively `-lstdc++`. What other unnecessary libraries are there?

Unfortunately, it is not enough.  See the comment about `CMAKE_CXX_IMPLICIT_LINK_LIBRARIES`: the C++ library is added into this var automatically by CMake.  I did not want to mess with this var, but that is another solution, i.e. to remove `c++` and `stdc++` from it.  I think `--as-needed` is a more clean solution, and it will cover other libs, e.g. I believe I saw `pthreads` being linked, while it was not needed.

> If this is for the shared library, please only apply it to the shared libary. This can be done by moving it into the `build_shared` section.
> 
> `--as-needed` is a positional option: It only applies to libraries listed after it. I don't know whether CMake/gcc/clang guarantee that `LINK_FLAGS` will be inserted before all the libraries. I'd avoid it if not necessary.

Thanks! I will move the code around.  Good point about the position of the option: it works now, but I will check if I can do it more robust.

https://github.com/llvm/llvm-project/pull/130856