tcg/s390: fix branch target change during code retranslation

Make sure to not modify the branch target. This ensure that the
branch target is not corrupted during partial retranslation.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Tested-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
