translate-all: consolidate tb init in tb_gen_code

We are partially initializing tb in tb_alloc. Instead, fully
initialize it in tb_gen_code, which is tb_alloc's only caller.

This saves an unnecessary write to tb->cflags.

Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1497038122-26364-1-git-send-email-cota@braap.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
