signal: added a wrapper for sigprocmask function

Create a wrapper for signal mask changes initiated by the guest;
(this includes syscalls and also the sigreturns from signal.c)
this will give us a place to put code which prevents the guest
from changing the handling of signals used by QEMU itself
internally.

The wrapper is called from all the guest-initiated sigprocmask, but
is not called from internal qemu sigprocmask calls.

Signed-off-by: Alex Barcelo <abarcelo@ac.upc.edu>
[PMM: Added calls to wrapper for sigprocmask uses in signal.c
when setting the signal mask on entry and exit from signal
handlers, since these also are guest-provided signal masks.]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
