os: implement waitable management. ...

This implements waitable management for Events (and
implements Events). It also refactors PM to use new
Event/Waitable semantics, and also adds STS_ASSERT
as a macro for asserting a boolean expression. The
rest of stratosphere has been refactored to use
STS_ASSERT whenever possible.

ams: initial support for 9.0.0

git subrepo clone https://github.com/Atmosphere-NX/libstratosphere stratosphere/libstratosphere ...

subrepo:
  subdir:   "stratosphere/libstratosphere"
  merged:   "0c5dab80"
upstream:
  origin:   "https://github.com/Atmosphere-NX/libstratosphere"
  branch:   "master"
  commit:   "0c5dab80"
git-subrepo:
  version:  "0.4.0"
  origin:   "https://github.com/ingydotnet/git-subrepo"
  commit:   "5d6aba9"