spirv_builder feature compile_codegen to disable building rustc_codegen_spirv

[Firestar99]

See writeup at Rust-GPU/cargo-gpu#69

breaking change:

• default MetadataPrintout changed from Full to None (controls printing of cargo::rerun-if-changed="path/to/file" for build script)

[Firestar99]

compile_codegen: include_str! all target_spec jsons for cargo gpu

this commit made my cargo fmt slow to an absolute crawl, needs investigation

ignore that, my IDE was just derpy I guess

[schell]

This is looking great @Firestar99 , what's left?

[Firestar99]

I'd wait and merge this PR together with Rust-GPU/cargo-gpu#69, in case I find something that still needs adjustment.

[schell]

Great!

[Firestar99]

Something I forgot to mention here: The default MetadataPrintout changed from Full to None, so you now have to explicitly enable it to get cargo::rerun-if-changed="path/to/file" messages