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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

At least, could you add to the comment that this is for the shared library only? For the static library, the linker is not even used (CMAKE_AR is being used) and LINKER_LANGUAGE/LINK_FLAGS being ignored.

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