Expand "show disassembler-options" output

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.