spirv_atomic: Define U32x2 storage buffers for 64-bit storage atomics

Some drivers do not support 64-bit atomics, and fallback to atomically modifying U32x2 vectors. This change ensures that U32x2 storage vectors are defined in the spir-v shader when 64-bit atomics are used.

Fixes a hang on some devices, notably Intel GPUs, when booting Pokemon Legends Arceus