Using do_sigprocmask directly was incorrect, as it will
leave the signal blocked by the outer layers of linux-user.

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211221025012.1057923-8-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>