longjmp(3) t_sigstack: Use a sigaltstack per handler entry.

longjmp evidently doesn't reset the state of whether the process is
executing on the alternate signal stack.  So when we re-enter the
signal handler, the alternate stack appears to be still in use, and
the system chooses the original stack for the second call to the
signal handler -- which trips our assertion asking to verify that the
signal handler is always using an alternate stack.

Not strictly necessary for the signal handler to use an alternate
stack on re-entry, but this makes it clearer that the signal handler
itself is always using the alternate stack so we can verify that the
interrupted code is _not_ in the signal handler.

With this change, the test now passes on aarch64.

PR lib/57946

(riastradh)