tcg: Sanity check goto_tb input

Checking that we don't try for idx != [01] is trivial.  Checking
that we don't issue more than one of any index requires a tad
more data and some ifdefs protecting that new variable.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
