[Migrated] We should replace `rustc_codegen_spirv::linker::test` unit tests with compiletest ones.

Issue automatically imported from old repo: https://github.com/EmbarkStudios/rust-gpu/issues/957
Old labels: t: enhancement,a: test
Originally creatd by eddyb on 2022-11-30T16:58:43Z

The main unique aspect of these tests is they take SPIR-V assembly as an input, not Rust code, e.g.:
https://github.com/EmbarkStudios/rust-gpu/blob/acb05d379982f35e6d4fbd85ff28af3e9876cf4c/crates/rustc_codegen_spirv/src/linker/test.rs#L185-L202

However, we might be able to use module_asm! to feed SPIR-V assembly into the compilation, and compiletest does have the ability to introduce dependencies to link against. The main weirdness we might need to deal with is all the definitions from e.g. core that we don't use, but DCE might be able to clean that up.

(Or we could even use e.g. extern "C" FFI in Rust code to describe such situations without module_asm! at all!)

If we can do this transition, we wouldn't have to deal with weird artificial compiler sessions and e.g.:

• #956

Start with crates/rustc_codegen_spirv/src/linker/test.rs, especially the tests around the referenced lines, and review the compiletest support and issue #956. Determine whether module_asm!, compiletest dependencies, or Rust FFI can represent the SPIR-V assembly inputs. Done means the linker tests use compiletest without the artificial compiler sessions.