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

[Michael Kruse via llvm-commits]

Meinersbur 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?
 
> I know that we have the test, but the library dependencies are still established by the linker, and they need to be satisfied. Which makes it awkward that we need to link c++ library for Fortran programs.

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.

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