workgroup_size / gl_WorkGroupSize support

[LegNeato]

I see this is commented out:

rust-gpu/crates/rustc_codegen_spirv/src/symbols.rs

Line 62 in 30024f8

// ("workgroup_size", WorkgroupSize), -- constant

Is there a way to get this info without hardcoding values? You can't include constants / variables in the attribute, so I see no way to have this in one place and instead you have to keep two places in sync manually.