rename tool from rust_gpu to rustc_codegen_spirv

[Firestar99]

As discussed with @eddyb, let's make our "tool" name match with our codegen backend name.

Important since we'll have glam refer to our rustc_codegen_spirv directly instead of going via our spirv() proc macro, which expands to our tool name.

[Firestar99]

@eddyb small problem:

error: attributes starting with `rustc` are reserved for use by the `rustc` compiler

Any other name suggestions than rustc_codegen_spirv?

• keep rust_gpu
• codegen_spirv