target/loongarch: Extract set_pc() helper

Signed-off-by: Jiajie Chen <c@jia.je>
Co-authored-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20230822032724.1353391-6-gaosong@loongson.cn>
[PMD: Extract helper from bigger patch]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230822071405.35386-9-philmd@linaro.org>