spirv-builder: force a single CGU (codegen-unit).

[eddyb]

The main reason rustc has a concept of "codegen units" is to allow running LLVM optimizations in parallel.
We have no such parallelism right now, so all CGUs do for us is cause wasteful duplication of work.
(this came up in other work where it caused a massive explosion of debuginfo)

I ran this to get the data in the table below (both before and after the change):

rm -rf target/spirv-builder/
cargo run -p example-runner-wgpu --release --no-default-features --features use-installed-tools -- -s Mouse
dust -csbrn10 target/spirv-builder/spirv-unknown-vulkan1.1

Before this PR (many CGUs)	After this PR (1 CGU)
128M └─┬ spirv-unknown-vulkan1.1 128M └─┬ release 117M ├─┬ deps 44M │ ├── libcore-f3590347654738e9.rlib 38M │ ├── libcore-f3590347654738e9.rmeta 6.2M │ ├── libglam-07642c7dac08746c.rlib 5.9M │ └── libglam-07642c7dac08746c.rmeta 10M └─┬ incremental 7.9M └─┬ spirv_std-3rzoyk5mydowb 7.9M └─┬ s-gk2fjiu00l-scpc61-tw2x301r7v8h 5.5M └── dep-graph.bin	123M └─┬ spirv-unknown-vulkan1.1 123M └─┬ release 112M ├─┬ deps 43M │ ├── libcore-b338bddeac41658a.rlib 38M │ ├── libcore-b338bddeac41658a.rmeta 6.2M │ ├── libglam-3dba180d0288b28f.rlib 5.9M │ └── libglam-3dba180d0288b28f.rmeta 10M └─┬ incremental 7.8M └─┬ spirv_std-3dftucm2yp1ij 7.8M └─┬ s-gk2fozx3vf-108z0x5-3fc8kftdgx7kt 5.5M └── dep-graph.bin