Cygwin: restore two instances of __stdcall

In the previous commit, __stdcall was removed from _dll_crt0 in
winsup.h and dcrt0.cc but not in lib/cygwin_crt0.c.  For consistency,
restore the first two occurrences of __stdcall.  We could instead
remove it from the declaration in lib/cygwin_crt0.c, but this might
appear to affect binary compatibility, even though it really doesn't.

winsup/cygwin/dcrt0.cc

@@ -1013,7 +1013,7 @@ __cygwin_exit_return:			\n\
 ");
 }
 
-extern "C" void
+extern "C" void __stdcall
 _dll_crt0 ()
 {
   /* Starting with Windows 10 rel 1511, the main stack of an application is

winsup/cygwin/winsup.h

@@ -147,7 +147,7 @@ extern int cygserver_running;
 class per_process;
 /* cygwin .dll initialization */
 void dll_crt0 (per_process *) __asm__ (_SYMSTR (dll_crt0__FP11per_process));
-extern "C" void _dll_crt0 ();
+extern "C" void __stdcall _dll_crt0 ();
 void dll_crt0_1 (void *);
 void dll_dllcrt0_1 (void *);