SPIR-V atomic operations on Metal

[kvark]

TL;DR: what SPIRV-Cross does today for atomics isn't guaranteed to work on Metal.

Context from WebGPU:

• https://github.com/gpuweb/gpuweb/issues/1360
• https://github.com/gpuweb/gpuweb/issues/2229
• https://github.com/gpuweb/gpuweb/issues/2377