When multiple threads concurrently use a function which has a static
variable with a constructor, gcc guarantees that only one thread will run
the constructor, and the other ones waits.  It uses __cxa_guard_acquire()/
release()/abort() for implementing this guarantee. Unfortunately these
functions, implemented in the C++ standard library, use the Linux futex
syscall which we don't implement. The futex system call is only used in
the rare case of contention - in the normal case, the __cxa_guard_*
functions uses just atomic memory operations.

This patch implements the bare minimum we need from futex to make the
__cxa_guard_* functions work: All we need is FUTEX_WAIT and FUTEX_WAKE,
with 0 timeout in FUTEX_WAIT and wake all in FUTEX_WAKE.

It's nice how the new waitqueues fit this implementation like glove to
a hand: We could use condvars too, but we anyway need to do the wake()
with the mutex locked (to get the futex's atomic test-and-wait), and in
that case we can use waitqueues without their additional internal mutex.

This patch also adds a test for this bug. Catching this bug in a real
application is very rare, but the test exposes it by defining an function-
static object with a very slow constructor (it sleeps for a second), and
calls the function from several threads concurrently.
Before this patch the test fails with:
   constructing
   syscall(): unimplemented system call 202. Aborting.

Fixes #199.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>