Gate spsc under the cas feature (Fixes #271)

[Kriskras99]

The spsc module should only be built if the cas feature is enabled.
This fixes issue #271.

[TDHolmes]

hmm, but spsc doesn't actually require cas operations. What we really need I think is to make atomic-polyfill a requirement for targets that have no atomic support (i.e., have full_atomic_polyfill cfg set). Does dropping optional = true like below fix it?

[target.riscv32i-unknown-none-elf.dependencies]
 atomic-polyfill = { version = "0.1.4" }

 [target.riscv32imc-unknown-none-elf.dependencies]
 atomic-polyfill = { version = "0.1.4" }

[Kriskras99]

I would suggest adding an atomic load/store gate and hide it behind that.
See 2a8f38c.
In this way people who don't care for atomic types don't have to pull in 12 extra dependencies.

[Kriskras99]

I have created an alternative pull request in #273 .
@TDHolmes you're solution also works but I'd like to avoid pulling in atomic_polyfill if I can.