Better implementation of GetLastError. (#20623)

Instead of calling the dynamically loaded kernel32.GetLastError, we can extract it from the TEB.
As shown by [Wine](34b1606019/include/winternl.h (L439)), the last error lives at offset 0x34 of the TEB in 32-bit Windows and at offset 0x68 in 64-bit Windows.

lib/std/Thread/Condition.zig

@@ -178,7 +178,7 @@ const WindowsImpl = struct {
 
         // Return error.Timeout if we know the timeout elapsed correctly.
         if (rc == os.windows.FALSE) {
-            assert(os.windows.kernel32.GetLastError() == .TIMEOUT);
+            assert(os.windows.GetLastError() == .TIMEOUT);
             if (!timeout_overflowed) return error.Timeout;
         }
     }