linux-user: Introduce host_signal_mask

Do not directly access the uc_sigmask member.
This is preparation for a sparc64 fix.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
12 files changed