Add support for retrieving the thread ID on DragonFly BSD (#106938)

Author: brad0

llvm/lib/Support/Unix/Threading.inc

@@ -29,7 +29,7 @@
 
 #include <pthread.h>
 
-#if defined(__FreeBSD__) || defined(__OpenBSD__)
+#if defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__DragonFly__)
 #include <pthread_np.h> // For pthread_getthreadid_np() / pthread_set_name_np()
 #endif
 
@@ -121,7 +121,7 @@ uint64_t llvm::get_threadid() {
     return InitSelf;
   }();
   return Self;
-#elif defined(__FreeBSD__)
+#elif defined(__FreeBSD__) || defined(__DragonFly__)
   return uint64_t(pthread_getthreadid_np());
 #elif defined(__NetBSD__)
   return uint64_t(_lwp_self());