[gdb/build] Fix Wstringop-truncation in coff_getfilename

[gdb/build] Fix Wstringop-truncation in coff_getfilename

When building gdb with -O2 -fsanitize-threads, I ran into
a Werror=stringop-truncation.

The problem is here in coff_getfilename in coffread.c:
...
      strncpy (buffer, aux_entry->x_file.x_n.x_fname, FILNMLEN);
      buffer[FILNMLEN] = '\0';
...

The constant FILNMLEN is expected to designate the size of
aux_entry->x_file.x_n.x_fname, but that's no longer the case since commit
60ebc257517 ("Fixes a buffer overflow when compiling assembler for the MinGW
targets.").

Fix this by using "sizeof (aux_entry->x_file.x_n.x_fname)" instead.

Likewise in xcoffread.c.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR build/30669
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30669