winpthreads: Try loading SetThreadDescription from kernel32.dll and fallback to kernelbase.dll

The SetThreadDescription function should be called from the kernel32.dll library.
Document why the fallback to kernelbase.dll is needed for compatibility purposes.

mingw-w64-libraries/winpthreads/src/misc.c

diff --git a/mingw-w64-libraries/winpthreads/src/misc.c b/mingw-w64-libraries/winpthreads/src/misc.c
index d000dde..b2d5ceb 100644
--- a/mingw-w64-libraries/winpthreads/src/misc.c
+++ b/mingw-w64-libraries/winpthreads/src/misc.c
@@ -55,6 +55,9 @@
         _pthread_get_tick_count_64 =
             (ULONGLONG (WINAPI *)(VOID))(void*) GetProcAddress(mod, "GetTickCount64");
 
+        _pthread_set_thread_description =
+            (HRESULT (WINAPI *)(HANDLE, PCWSTR))(void*) GetProcAddress(mod, "SetThreadDescription");
+
         /* <1us precision on Windows 10 */
         _pthread_get_system_time_best_as_file_time =
             (void (WINAPI *)(LPFILETIME))(void*) GetProcAddress(mod, "GetSystemTimePreciseAsFileTime");
@@ -64,11 +67,18 @@
         /* >15ms precision on Windows 10 */
         _pthread_get_system_time_best_as_file_time = GetSystemTimeAsFileTime;
 
-    mod = GetModuleHandleA("kernelbase.dll");
-    if (mod)
+    /* Although SetThreadDescription lives in kernel32.dll, on Windows Server 2016,
+     * Windows 10 LTSB 2016 and Windows 10 version 1607, it was only available in
+     * kernelbase.dll. So, load it from there for maximum coverage.
+     */
+    if (!_pthread_set_thread_description)
     {
-        _pthread_set_thread_description =
-            (HRESULT (WINAPI *)(HANDLE, PCWSTR))(void*) GetProcAddress(mod, "SetThreadDescription");
+        mod = GetModuleHandleA("kernelbase.dll");
+        if (mod)
+        {
+            _pthread_set_thread_description =
+                (HRESULT (WINAPI *)(HANDLE, PCWSTR))(void*) GetProcAddress(mod, "SetThreadDescription");
+        }
     }
 }
 #if defined(__GNUC__) && __GNUC__ >= 9 && !defined(__clang__)