[openmp] [test] Set the right calling convention for the Windows thread start function

[mstorsjo]

Summary

This is required on i386 Windows; this fixes 99 testcases
in that build configuration.

openmp/runtime/test/omp_testsuite.h

 
 // encapsulates the information about a pthread-callable function
 struct thread_func_info_t {
   void* (*start_routine)(void*);
   void* arg;
 };
 
 // call the void* start_routine(void*);
-static DWORD __thread_func_wrapper(LPVOID lpParameter) {
+static DWORD WINAPI __thread_func_wrapper(LPVOID lpParameter) {
   struct thread_func_info_t* function_information;
   function_information = (struct thread_func_info_t*)lpParameter;
   function_information->start_routine(function_information->arg);
   free(function_information);
   return 0;
 }
 
 // attr is ignored