tcg: Change translator-side labels to a pointer

This is improved type checking for the translators -- it's no longer
possible to accidentally swap arguments to the branch functions.

Note that the code generating backends still manipulate labels as int.

With notable exceptions, the scope of the change is just a few lines
for each target, so it's not worth building extra machinery to do this
change in per-target increments.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Cc: Michael Walle <michael@walle.cc>
Cc: Leon Alrae <leon.alrae@imgtec.com>
Cc: Anthony Green <green@moxielogic.com>
Cc: Jia Liu <proljc@gmail.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Blue Swirl <blauwirbel@gmail.com>
Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Richard Henderson <rth@twiddle.net>
