make sigaltstack() compatible with swapcontext()

The following patches make it possible to use swapcontext() in a sighandler that works on sigaltstack.
The approach is inspired by Andy Lutomirski's suggestion that sigaltstack should disarm itself after saving into uc_stack:
https://lkml.org/lkml/2016/2/1/594

I add the SS_AUTODISARM flag that does exactly that. On sighandler exit, the sigaltstack is restored from uc_stack. Another possible name could be SS_ONESHOT, but, since it gets always re-enabled, I choose SS_AUTODISARM.

Change since v5:
- Fix description of patch 4/4

Change since v4:
- Implement this Andy Lutomirski's suggestion:
https://lkml.org/lkml/2016/3/6/158
that allows the run-time probing of the existence of the flags added in the future.

[PATCH 1/4] [Cleanup] x86: signal: unify the sigaltstack check with
A clean-up patch that unifies x86's sigaltstack handling with other arches.
[PATCH 2/4] sigaltstack: preparations for adding new SS_xxx flags
Andy's suggested changes
[PATCH 3/4] sigaltstack: implement SS_AUTODISARM flag
This patch implements SS_AUTODISARM flag
[PATCH 4/4] selftests: Add test for
This patch adds the selftest code for new functionality

Diffstat:

 arch/x86/kernel/signal.c                     |   23 +--
 include/linux/sched.h                        |    8 +
 include/linux/signal.h                       |    4 
 include/uapi/linux/signal.h                  |    7 +
 kernel/fork.c                                |    2 
 kernel/signal.c                              |   26 ++--
 tools/testing/selftests/Makefile             |    1 
 tools/testing/selftests/sigaltstack/Makefile |    8 +
 tools/testing/selftests/sigaltstack/sas.c    |  156 +++++++++++++++++++++++++++
 9 files changed, 208 insertions(+), 27 deletions(-)