WIP compiler: implement atomic operations in package sync/atomic

[aykevl]

Implement functions in package sync/atomic using LLVM intrinsics.
Does not work yet:

• the compiler emits unresolved symbols like __sync_fetch_and_add_8 on Cortex-M, we need to implement something for that
• atomic operations do not seem to be implemented in the WebAssembly backend: it crashes with a nasty LIBCLANG FATAL ERROR: Cannot select error
• some functions need to be added to go-llvm.

I am making the PR anyway for future reference.

[aykevl]

Also see: https://reviews.llvm.org/D61034

[aykevl]

A patch that implements some atomic builtins: https://reviews.llvm.org/D61052

With the right patch to go-llvm, tests pass locally. I'll wait and see what happens with the upstream patches.

[aykevl]

TODO: remove src/runtime/atomic.go.

[deadprogram]

Seems like the upstream patch to LLVM was tagged "Abandoned".

Any chance of getting this feature anyhow?

[aykevl]

Yes, this should be implemented on the TinyGo side because compiler-rt cannot know what to do in all cases. For example, none of the Cortex-M devices support 64-bit atomic stores so it should disable interrupts, but interrupts cannot be disabled in all cases because it can affect interrupt latency.

[aykevl]

Replaced by #1113.