Add libc::consts::os::posix01::PTHREAD_STACK_MIN

Represents the minimum size of a thread's stack.  As such, it's both
platform and architecture-specific.

I put it under posix01 even though it predates POSIX.1-2001 by some
years.  I believe it was first formalized in SUSv2.  I doubt anyone
cares, though.
Retry on EINVAL from pthread_attr_setstacksize()

Enforce that the stack size is > RED_ZONE + PTHREAD_STACK_MIN.  If the
call to pthread_attr_setstacksize() subsequently fails with EINVAL, it
means that the platform requires the stack size to be a multiple of the
page size.  In that case, round up to the nearest page and retry.

Fixes #11694.
Use __pthread_get_minstack() when available.

glibc >= 2.15 has a __pthread_get_minstack() function that returns
PTHREAD_STACK_MIN plus however many bytes are needed for thread-local
storage.  Use it when it's available because just PTHREAD_STACK_MIN is
not enough in applications that have big thread-local storage
requirements.

Fixes #6233.
Introduce marker types for indicating variance and for opting out
of builtin bounds.

Fixes #10834.
Fixes #11385.
cc #5922.
auto merge of #11885 : bnoordhuis/rust/issue11694, r=alexcrichton

EINVAL means that the requested stack size is either not a multiple
of the system page size or that it's smaller than PTHREAD_STACK_MIN.
Figure out what the case is, fix it up and retry.  If it still fails,
give up, like before.

Suggestions for future improvements:

  * don't fail!() but instead signal a condition, or
  * silently ignore the error and use a default sized stack.

Fixes #11694.

The first two commits put the framework in place, the third one contains the meat.
auto merge of #11768 : nikomatsakis/rust/issue-11385-cell-and-variance, r=pnkfelix

Introduce marker types for indicating variance and for opting out
of builtin bounds.

Fixes #10834.
Fixes #11385.
cc #5922.

r? @pnkfelix (since you reviewed the variance inference in the first place)
Add an AtomicU64 type to std::sync::atomics

This also generalizes all atomic intrinsics over T so we'll be able to add u8
atomics if we really feel the need to (do we really want to?)
Enable the +v7 feature on Android by default

With the recently added double word CAS functionality on 32-bit ARM (enabled via
a 64-bit atomic instruction in LLVM IR), without some extra features enabled
LLVM lowers code to function calls which emulate atomic instructions. With the
v7 feature enabled, proper 64-bit CAS instructions are used on 32-bit arm.

I've been told that v7 for arm is what we should have been doing anyway. This is
overridable by providing some other non-empty feature string.
extra: Make room for more sync primitives
extra: Add an intrusive MPSC to be used soon
std: Hardcode pthread constants and structures

This allows for easier static initialization of a pthread mutex, although the
windows mutexes still sadly suffer.

Note that this commit removes the clone() method from a mutex because it no
longer makes sense for pthreads mutexes. This also removes the Once type for
now, but it'll get added back shortly.
extra: Re-add the Once primitve to extra::sync

This originally lived in std::unstable::mutex, but now it has a new home (and a
more proper one).
std: Remove try_send_deferred plus all fallout

Now that extra::sync primitives are built on a proper mutex instead of a
pthreads one, there's no longer any use for this function.
extra: Introduce a mutex type for native/green threads
Various bug fixes and rebase conflicts