Fix Hurd getcwd build with GCC >= 13

The build of glibc for i686-gnu has been failing for a while with GCC
mainline / GCC 13:

../sysdeps/mach/hurd/getcwd.c: In function '__hurd_canonicalize_directory_name_internal':
../sysdeps/mach/hurd/getcwd.c:242:48: error: pointer 'file_name' may be used after 'realloc' [-Werror=use-after-free]
  242 |                   file_namep = &buf[file_namep - file_name + size / 2];
      |                                     ~~~~~~~~~~~^~~~~~~~~~~
../sysdeps/mach/hurd/getcwd.c:236:25: note: call to 'realloc' here
  236 |                   buf = realloc (file_name, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~

Fix by doing the subtraction before the reallocation.

Tested with build-many-glibcs.py for i686-gnu.

[samuel.thibault@ens-lyon.rg: Removed mention of this being a bug]

Message-Id: <18587337-7815-4056-ebd0-724df262d591@codesourcery.com>