Do not adjust mtime timezone on Windows

PR win32/25302 notes that gdb will crash when trying to "run" even a
simple program on Windows.  The essential bug here is that the BFD
cache can easily be corrupted -- I have sent a separate patch for
that.

The particular reason that the cache is corrupted on Windows is that
gnulib overrides "stat" to make it do timezone adjustment -- but BFD
does not use this version of stat.  The difference here triggers the
latent cache bug, but can also cause other bugs as well; in particular
it can cause spurious warnings about source files being newer.

This patch simply removes the stat override on mingw, making gnulib
and BFD agree.

I tested this by backing out the local AdaCore changes to work around
this bug and then verifying that I could reproduce it.  Then, I
applied this patch and verified that "run" works again.

2020-09-08  Tom Tromey  <tromey@adacore.com>

	PR win32/25302:
	* update-gnulib.sh: Apply stat patch.
	* patches/0001-use-windows-stat: New file.
	* import/m4/stat.m4: Update.
	* configure: Rebuild.

gnulib/configure

@@ -26907,8 +26907,7 @@ $as_echo "#define ssize_t int" >>confdefs.h
 
   case "$host_os" in
     mingw*)
-                  REPLACE_STAT=1
-      ;;
+                        ;;
     *)
                         { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether stat handles trailing slashes on files" >&5
 $as_echo_n "checking whether stat handles trailing slashes on files... " >&6; }