gas: Handle bad -gdwarf options, just like bad --gdwarf options.

parse_args uses getopt_long_only so it can handle long options both
with double and single dash. But this means that some single dash
options like -gdwarf-1 don't generate an error (unlike --gdwarf-1).

This is especially confusing since there is also --gdwarf2, but no
--gdwarf4 (it is --gdwarf-4). When giving -gdwarf4 the option is
silently interpreted as -g (which set dwarf_version to 2). This causes
some confusion for people who don't expect this and suddenly get
DWARF2 instead of DWARF4 as they might expect.

So make it so that the -gdwarf<unknown> creates an error, just like
--gdwarf<unknown> would.

gas/ChangeLog

@@ -1,3 +1,7 @@
+2020-08-26  Mark Wielaard  <mark@klomp.org>
+
+	* as.c (parse_args): Handle bad -gdwarf options.
+
 2020-08-28  Cooper Qu  <cooper.qu@linux.alibaba.com>
 
 	* gas/config/tc-csky.c (md_begin): Set attributes.

gas/as.c

@@ -816,6 +816,13 @@ This program has absolutely no warranty.\n"));
 	      && md_parse_option (optc, optarg))
 	    continue;
 
+	  /* We end up here for any -gsomething-not-already-a-long-option.
+	     give some useful feedback on not (yet) supported -gdwarfxxx
+	     versions/sections/options.  */
+	  if (strncmp (old_argv[optind - 1], "-gdwarf",
+		       strlen ("-gdwarf")) == 0)
+	    as_fatal (_("unknown DWARF option %s\n"), old_argv[optind - 1]);
+
 	  if (md_debug_format_selector)
 	    debug_type = md_debug_format_selector (& use_gnu_debug_info_extensions);
 	  else if (IS_ELF)