[Flang] explicitly link the pthread library when building shared flang-rt.

Author: DanielCChen

flang-rt/cmake/modules/AddFlangRT.cmake

@@ -142,6 +142,8 @@ function (add_flangrt_library name)
   endif ()
   if (build_shared)
     add_library("${name_shared}" SHARED ${extra_args} ${ARG_ADDITIONAL_HEADERS} ${ARG_UNPARSED_ARGUMENTS})
+    find_package(Threads REQUIRED)
+    target_link_libraries(${name_shared} PUBLIC Threads::Threads)
   endif ()
 
   if (libtargets)