allows users to atomically unmask, switch stacks,
and return from a signal context.
The thread's signal mask and stack status are
restored from the context structure pointed to by
The system call does not return;
the users stack pointer, frame pointer, argument pointer,
and processor status longword are restored from the context.
Execution resumes at the specified pc.
This system call is used by the trampoline code and
when returning from a signal to the previously executing program.