gdb: adjust x_file fields on COFF readers

Commit e86fc4a5 ("PR 28447: implement multiple parameters for .file
on XCOFF") changes the structure associated to the internal
representation of files in COFF formats.  However, gdb directory update
has been forgotten, leading to compilation errors of this kind:

      CXX    coffread.o
    /home/simark/src/binutils-gdb/gdb/coffread.c: In function 'const char* coff_getfilename(internal_auxent*)':
    /home/simark/src/binutils-gdb/gdb/coffread.c:1343:29: error: 'union internal_auxent::<unnamed struct>::<unnamed>' has no member named 'x_zeroes'
     1343 |   if (aux_entry->x_file.x_n.x_zeroes == 0)
          |                             ^~~~~~~~

Fix it by adjusting the COFF code in GDB.

Change-Id: I703fa134bc722d47515efbd72b88fa5650af6c3c