Implement a Once primitive for initialization

Of the 8 static mutexes that are currently in-use by the compiler and its
libraries, 4 of them are currently used for one-time initialization. The
unforunate side effect of using a static mutex is that the mutex is leaked.

This primitive should provide the basis for efficiently keeping track of
one-time initialization as well as ensuring that it does not leak the internal
mutex that is used.

I have chosen to put this in libstd because libstd is currently making use of a
static initialization mutex (rt::local_ptr), but I can also see a more refined
version of this type being suitable to initialize FFI bindings (such as
initializing LLVM and initializing winsock networking on windows). I also intend
on adding "helper threads" to libnative, and those will greatly benefit from a
simple "once" primitive rather than always reinventing the wheel by using
mutexes and bools.

I would much rather see this primitive built on a mutex that blocks green
threads appropriately, but that does not exist at this time, so it does not
belong outside of `std::unstable`.
Convert relevant static mutexes to Once
auto merge of #11187 : alexcrichton/rust/once, r=brson

Rationale can be found in the first commit, but this is basically the same thing as `pthread_once`