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

[Slava Zakharin via llvm-commits]

vzakhari wrote:

> I don't it does what you think it does. The static library is just a collection of object files. Only when linking the executable you actually know what "is needed". Until then, you want all the object files in the library.
> 
> We actually have a test that checks that no libc++ dependencies are required: https://github.com/llvm/llvm-project/blob/main/flang-rt/test/Runtime/no-cpp-dep.c

It works for the shared libraries build, I checked.

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.

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