cpus.c: Use pthread_sigmask() rather than sigprocmask()

cpus.c: Use pthread_sigmask() rather than sigprocmask()

On Linux, sigprocmask() and pthread_sigmask() are in practice the
same thing (they only set the signal mask for the calling thread),
but the documentation states that the behaviour of sigprocmask() in a
multithreaded process is undefined. Use pthread_sigmask() instead
(which is what we do in almost all places in QEMU that alter the
signal mask already).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1463420039-29761-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>