instrs: Add support for undocumented instructions

Undocumented instruction are not decoded by default.

- SALC: undocumented in any recent manual and unsupported by newer
  Intel CPUs. Including as listed by [1,2].
- Undocumented FPU instructions: see [2].

[1]: http://www.rcollins.org/secrets/opcodes/SALC.html
[2]: https://github.com/xoreaxeaxeax/sandsifter/issues/33

meson.build

@@ -35,6 +35,9 @@ endif
 if get_option('archmode') != 'only32'
   generate_args += ['--64']
 endif
+if get_option('with_undoc')
+  generate_args += ['--with-undoc']
+endif
 
 instr_data = custom_target('tables',
                            command: [python3, '@INPUT0@', '@INPUT1@', '@OUTPUT@'] + generate_args,