[llvm] [Flang] explicitly link the pthread library when building shared flang-rt. (PR #129956)

[Michael Kruse via llvm-commits]

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

I expected it to be trivially present on Windows, as part of the Win32 CRT (there is a `CMAKE_USE_WIN32_THREADS_INIT` results variable).

Aynway, LGTM

https://github.com/llvm/llvm-project/pull/129956