[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

gdb/coffread.c

diff --git a/gdb/coffread.c b/gdb/coffread.c
index 33fb2ba..6ec341c 100644
--- a/gdb/coffread.c
+++ b/gdb/coffread.c
@@ -1371,8 +1371,9 @@ coff_getfilename (union internal_auxent *aux_entry)
     }
   else
     {
-      strncpy (buffer, aux_entry->x_file.x_n.x_fname, FILNMLEN);
-      buffer[FILNMLEN] = '\0';
+      size_t x_fname_len = sizeof (aux_entry->x_file.x_n.x_fname);
+      strncpy (buffer, aux_entry->x_file.x_n.x_fname, x_fname_len);
+      buffer[x_fname_len] = '\0';
     }
   result = buffer;
 

gdb/xcoffread.c

diff --git a/gdb/xcoffread.c b/gdb/xcoffread.c
index 1538d1c..8930cf1 100644
--- a/gdb/xcoffread.c
+++ b/gdb/xcoffread.c
@@ -1598,8 +1598,9 @@ coff_getfilename (union internal_auxent *aux_entry, struct objfile *objfile)
 		     + aux_entry->x_file.x_n.x_n.x_offset));
   else
     {
-      strncpy (buffer, aux_entry->x_file.x_n.x_fname, FILNMLEN);
-      buffer[FILNMLEN] = '\0';
+      size_t x_fname_len = sizeof (aux_entry->x_file.x_n.x_fname);
+      strncpy (buffer, aux_entry->x_file.x_n.x_fname, x_fname_len);
+      buffer[x_fname_len] = '\0';
     }
   return (buffer);
 }