Switch rustc_codegen_spirv to static lib

[repi]

Was set as a dylib but not really sure why. Having it as a static library enables dependencies, like spriv-builder, to start building directly after the frontend section of the build of the crate is done. Rather than having to wait for the codegen as well.

Saw clean ´spirv-builder` build times go from 46.3 -> 45.8 seconds with this. Have seen bigger benefits when combined with other optimizations as well (example: #858 (comment))

Part of #858

[repi]

Interesting CI failure, so we do need this as dylib for the (doc) tests?

Oh wait doesn't it have to be dylib for rustc to load the codegen plugin dynamically? Now I remember :) Though not sure how it could have worked to build anything for me locally with this then, I did do a cargo clean before hand and examples did work - but maybe it still hadn't properly cleaned out a previously built .dylib and used that still

[repi]

That must have indeed been the case, as can't reproduce this building without being a dylib on a separate machine after properly clean build.

[eddyb]

I did do a cargo clean before hand and examples did work

Heh, I don't trust tools to do this properly (for whatever reason), and I rm -rf target/ instead.