linux-user/signal.c: Don't pass sigaction uninitialised sa_flags

When forcing a fatal signal, we weren't initialising the sa_flags
field in the struct sigaction we used to reset the signal handler
to SIG_DFL.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
