Expand "show disassembler-options" output

I typed this:

    (gdb) help set disassembler-options
    Set the disassembler options.
    Usage: set disassembler-options OPTION [,OPTION]...

    See: 'show disassembler-options' for valid option values.

... so I tried what it said and got:

    (gdb) show disassembler-options
    The current disassembler options are ''

This surprised me a little, so this patch adds some text to explain
the situation when an architecture does not have disassembler options.

While there I noticed one more spot where gdb was not using the GNU
style for metasyntactic variables.  This patch fixes this as well.

gdb/ChangeLog
2018-09-16  Tom Tromey  <tom@tromey.com>

	* disasm.c (show_disassembler_options_sfunc): Use GNU style for
	metasyntactic variables.  Print message if no disassembler options
	are available.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2018-09-16  Tom Tromey  <tom@tromey.com>
+
+	* disasm.c (show_disassembler_options_sfunc): Use GNU style for
+	metasyntactic variables.  Print message if no disassembler options
+	are available.
+
 2018-09-15  Tom Tromey  <tom@tromey.com>
 
 	* infcmd.c (get_inferior_args): Return const char *.

gdb/disasm.c

@@ -1008,19 +1008,24 @@ show_disassembler_options_sfunc (struct ui_file *file, int from_tty,
   if (options == NULL)
     options = "";
 
-  fprintf_filtered (file, _("The current disassembler options are '%s'\n"),
+  fprintf_filtered (file, _("The current disassembler options are '%s'\n\n"),
 		    options);
 
   valid_options_and_args = gdbarch_valid_disassembler_options (gdbarch);
 
   if (valid_options_and_args == NULL)
-    return;
+    {
+      fputs_filtered (_("There are no disassembler options available "
+			"for this architecture.\n"),
+		      file);
+      return;
+    }
 
   valid_options = &valid_options_and_args->options;
 
-  fprintf_filtered (file, _("\n\
+  fprintf_filtered (file, _("\
 The following disassembler options are supported for use with the\n\
-'set disassembler-options <option>[,<option>...]' command:\n"));
+'set disassembler-options OPTION [,OPTION]...' command:\n"));
 
   if (valid_options->description != NULL)
     {