Make some enhancements to atomic integers

[Kaiepi]

• Give nqp native refs to allow for usage of atomic ops in nqp.
• Introduce a semaphore-like Counter that performs comparably.
• I'm getting tired of writing every atomic op twice in Rakudo. Maybe it's time to give the JVM and JS backend atomic int ops?
• Let Rakudo building at all be the test of native refs, since they will be required at that point in time soon enough.
• Counter needs sanity tests in its own right. Swapping out Semaphore and its acquire/release for a Counter and its want_pred/need_succ in t/spec/S17-lowlevel/semaphore.t passes OK.
• Takes the place of https://github.com/Raku/nqp/pull/774.

[lizmat]

It would be really nice if someone could continue the work of Kaiepi

@patrickbkr @ab5tract @timo @niner