Rp2040 atomics

[Uthedris]

This adds the missing RP2040 atomic external definitions to support Zig's @atomicLoad, @atomicRmw, @aromicStore, @cmpxchgStrong and @cmpxchgWeak. operations.

[arkadiuszwojcik]

@Uthedris so _ = @import("hal/atomic.zig"); from hal.zig can be removed right now? I had to use it so conditional comptime export would work. Now chip check in that file is removes so maybe this line is not required?

[Uthedris]

@arkadiuszwojcik the guard against being in test mode still seems to be required. When I removed it, I could build my program that used atomics just fine, but the GitHub test suite failed the Unit Test Ports tests.