Remove core::sync::atomic::* from the HAL

[romancardenas]

As discussed in yesterday's WG meeting, saying that the E310x chip supports atomic operations is... arguable, as LR/WC operations are not supported at all. I think we should remove dependencies to the core::sync::atomic module to allow compilations with the riscv32imc target and use atomic emulation such as atomic-polyfill when needed. Users can still compile for riscv32imac and use atomics if they want (provided that they only use simple load/store operations).

[jamesmunns]

@romancardenas can this be closed now that #54 has been merged?

[romancardenas]

Yep, thanks!