Fix reporting of SS_ONSTACK

Fix reporting of SS_ONSTACK in nested signal delivery when sigaltstack()
is used on some architectures.

Add a unit test for this. I tested the test by introducing the bug
on amd64. I did not test it on other architectures.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D18347