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)
move concurrent stuff from libextra to libsync

one test fail