ams: update build system to add generic-fallback support

ams: add -Wno-format-truncation to flags

ams: centralize system thread definitions

hos: change initialization API ...

This was needed to make stratosphere buildable with debugging on.

os:: assertions rely on GetCurrentThread() working, and this requires
the global os resource manager to be constructed. However, __appInit executes
before global constructors. We now require that hos::InitializeForStratosphere()
be called before anything else is done. This initializes the os resource manager,
sets the hos version for libnx, and may do more things in the future.

TODO: Consider replacing __appInit/__appExit with ams:: namespace functions in general,
and wrap them so that we guarantee hos::InitializeForStratosphere is called first, and
generally ensure a consistent stratosphere environment.

erpt: reimplement the sysmodule (#875) ...

* erpt: reimplement the sysmodule

* fatal: update for latest bindings

* erpt: amend logic for culling orphan attachments