To stop the user program interfering with the bouncer's use of
SIGSYS, intercept and emulate the sigaction() and rt_sigprocmask()
syscalls to prevent userspace masking or handling SIGSYS.

This is of course impossible: signals are highly hostile to
proxying or emulation.  So only make basic efforts to make this
work.

It is assumed that the architecture has no sigaction(), signal()
or sigprocmask() syscalls that would provide a means to circumvent
this emulation.  arm64 is new enough not to have them.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>