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/import/m4/stat.m4

@@ -15,7 +15,7 @@ AC_DEFUN([gl_FUNC_STAT],
     mingw*)
       dnl On this platform, the original stat() returns st_atime, st_mtime,
       dnl st_ctime values that are affected by the time zone.
-      REPLACE_STAT=1
+      dnl REPLACE_STAT=1
       ;;
     *)
       dnl AIX 7.1, Solaris 9, mingw64 mistakenly succeed on stat("file/").