target/arm: Use tcg_constant_i64() in do_sat_addsub_64()

The immediate value used for comparison is constant and
read-only. Move it to the constant pool. This frees a
TCG temporary for unsigned saturation opcodes.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211029231834.2476117-5-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>