Move gdb_bfd_errmsg to gdb_bfd.c

gdb_bfd.c contains most of gdb's BFD-related utility functions.
However, gdb_bfd_errmsg is in utils.c.  It seemed better to me to move
this out of util.[ch] and into the BFD-related file instead.

Tested by rebuilding.

gdb/utils.c

@@ -3026,34 +3026,6 @@ gdb_argv::reset (const char *s)
   m_argv = argv;
 }
 
-#define AMBIGUOUS_MESS1	".\nMatching formats:"
-#define AMBIGUOUS_MESS2	\
-  ".\nUse \"set gnutarget format-name\" to specify the format."
-
-std::string
-gdb_bfd_errmsg (bfd_error_type error_tag, char **matching)
-{
-  char **p;
-
-  /* Check if errmsg just need simple return.  */
-  if (error_tag != bfd_error_file_ambiguously_recognized || matching == NULL)
-    return bfd_errmsg (error_tag);
-
-  std::string ret (bfd_errmsg (error_tag));
-  ret += AMBIGUOUS_MESS1;
-
-  for (p = matching; *p; p++)
-    {
-      ret += " ";
-      ret += *p;
-    }
-  ret += AMBIGUOUS_MESS2;
-
-  xfree (matching);
-
-  return ret;
-}
-
 /* Return ARGS parsed as a valid pid, or throw an error.  */
 
 int