gdb: Add non-enum disassembler options

This is paired with "opcodes: Add non-enum disassembler options".

There is a portable mechanism for disassembler options and used on some
architectures:

-   ARC
-   Arm
-   MIPS
-   PowerPC
-   RISC-V
-   S/390

However, it only supports following forms:

-   [NAME]
-   [NAME]=[ENUM_VALUE]

Valid values for [ENUM_VALUE] must be predefined in
disasm_option_arg_t.values. For instance, for -M cpu=[CPU] in ARC
architecture, opcodes/arc-dis.c builds valid CPU model list from
include/elf/arc-cpu.def.

In this commit, it adds following format:

-   [NAME]=[ARBITRARY_VALUE] (cannot contain "," though)

This is identified by NULL value of disasm_option_arg_t.values
(normally, this is a non-NULL pointer to a NULL-terminated list).

gdb/ChangeLog:

	* gdb/disasm.c (set_disassembler_options): Add support for
	non-enum disassembler options.
	(show_disassembler_options_sfunc): Likewise.

gdb/disasm.c

@@ -1270,6 +1270,8 @@ set_disassembler_options (const char *prospective_options)
 	    if (memcmp (opt, valid_options->name[i], len) != 0)
 	      continue;
 	    arg = opt + len;
+	    if (valid_options->arg[i]->values == NULL)
+	      break;
 	    for (j = 0; valid_options->arg[i]->values[j] != NULL; j++)
 	      if (disassembler_options_cmp
 		    (arg, valid_options->arg[i]->values[j]) == 0)
@@ -1391,6 +1393,8 @@ The following disassembler options are supported for use with the\n\
 
       for (i = 0; valid_args[i].name != NULL; i++)
 	{
+	  if (valid_args[i].values == NULL)
+	    continue;
 	  gdb_printf (file, _("\n\
   For the options above, the following values are supported for \"%s\":\n   "),
 		      valid_args[i].name);