Support more atomic operations

[FantasyVR]

Concisely describe the proposed feature If possible, I need to use more atomic operations like: atomicExch(), atomicCAS()

[0xzhang]

I would like to help! : ）

[lucywsq]

Hi, @0xzhang please join this slack channel, then we would respond to your problems on time. We can communicate faster as well.

[Yihao-Shi]

hello, does Taichi support atomicCAS now? I am trying to construct linear bvh using Taichi, but it won't be computationally efficient without it.