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_options.txt

@@ -1 +1,2 @@
 option('archmode', type: 'combo', choices: ['both', 'only32', 'only64'])
+option('with_undoc', type: 'boolean', value: false)