[llvm] [flang-rt] Fix weak RPC stub on Mac-OS (PR #200998)

[Leandro Lupori via llvm-commits]

https://github.com/luporl approved this pull request.

LGTM, thanks.

I can build and run flang-rt tests with no errors.
`iso_fortran_env_impl.f90` passes on my machine.

https://github.com/llvm/llvm-project/pull/200998