kernel: atomics: support for 64-bit atomic operations

[cfriedt]

Is your enhancement proposal related to a problem? Please describe.
64-bit architectures need 64-bit atomic operations but currently Zephyr's atomics are limited to 32-bit only.

Describe the solution you'd like
We should add support for 64-bit atomics.

Describe alternatives you've considered
Not having 64-bit atomics.. but that's why I'm submitting this enhancement issue.

Additional context
32 additional bits per atomic_t (but only for 64-bit builds).