MDEV-25602 get rid of __WIN__ in favor of standard _WIN32

This fixed the MySQL bug# 20338 about misuse of double underscore
prefix __WIN__, which was old MySQL's idea of identifying Windows
Replace it by _WIN32 standard symbol for targeting Windows OS
(both 32 and 64 bit)

Not that connect storage engine is not fixed in this patch (must be
fixed in "upstream" branch)

dbug/dbug.c

@@ -97,7 +97,7 @@ extern void __gcov_flush();
 #define fnmatch(A,B,C) strcmp(A,B)
 #endif
 
-#if defined(__WIN__)
+#if defined(_WIN32)
 #include <process.h>
 #endif
 
@@ -1872,7 +1872,7 @@ static void DoPrefix(CODE_STATE *cs, uint _line_)
     (void) fprintf(cs->stack->out_file->file, "%5d: ", cs->lineno);
   if (cs->stack->flags & TIMESTAMP_ON)
   {
-#ifdef __WIN__
+#ifdef _WIN32
     /* FIXME This doesn't give microseconds as in Unix case, and the resolution is
        in system ticks, 10 ms intervals. See my_getsystime.c for high res */
     SYSTEMTIME loc_t;
@@ -2203,7 +2203,7 @@ void _db_flush_()
 }
 
 
-#ifndef __WIN__
+#ifndef _WIN32
 void _db_suicide_()
 {
   int retval;
@@ -2222,7 +2222,7 @@ void _db_suicide_()
   fprintf(stderr, "sigsuspend returned %d errno %d \n", retval, errno);
   assert(FALSE); /* With full signal mask, we should never return here. */
 }
-#endif  /* ! __WIN__ */
+#endif  /* ! _WIN32 */
 
 
 void _db_lock_file_()