Do not emit a uint64_t, but a tcg_target_ulong, aka uintptr_t.
This reduces the size of the constant on 32-bit hosts.
The assert for label != NULL has to be removed because that
is a valid value for exit_tb.

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