accel/tcg: Move tb_lookup to cpu-exec.c

accel/tcg: Move tb_lookup to cpu-exec.c

Now that we've moved helper_lookup_tb_ptr, the only user
of tb-lookup.h is cpu-exec.c; merge the contents in.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>