Add atomic_shim to support platforms where 64-bit atomics are not available

[nazar554]

Add atomic_shim to support platforms where 64-bit atomics are not available. Closes #22

atomic-shim does target detection, so it is using std::atomic where possible.

[nazar554]

Seems that it will need lazy_static on MIPS or something like that:

error[E0015]: calls in statics are limited to constant functions, tuple structs and tuple variants
  --> /cargo/git/checkouts/quanta-6fa06bec5aded43c/26e996a/src/lib.rs:66:35
   |
66 | static GLOBAL_RECENT: AtomicU64 = AtomicU64::new(0);
   |                                   ^^^^^^^^^^^^^^^^^

ctor works as well with #[ctor] attribute (on 0.3 branch)

0.6 doesn't build on MIPS yet because of

error[E0433]: failed to resolve: could not find `arch` in `self`
  --> /cargo/registry/src/github.com-1ecc6299db9ec823/raw-cpuid-8.1.1/src/lib.rs:50:37
   |
50 |         let result = unsafe { self::arch::__cpuid_count(a, c) };
   |                                     ^^^^ could not find `arch` in `self`

[tobz]

@nazar554 Can you port the #[ctor] change from your feature branch into this one so we can keep constness?

[nazar554]

@tobz Done, I also added #[cfg] checks around raw-cpuid so it actually builds on MIPS with cross

[tobz]

One small issue then I think we're good to merge.