Use rustc_codegen_ssa

[bjorn3]

Introduced in rust-lang/rust#55627

This will reduce duplication between rustc_codegen_llvm and rustc_codegen_cranelift and will make it easier to implement not yet implemented stuff.

Blocked on rust-lang/rust#56048 (rustc_codegen_ssa didn't get included in the sysroot)

I am working on this in the wip_use_cg_ssa branch.

[bjorn3]

Now that rust-lang/rust#56048 is merged. f4ae9a4 starts using rustc_codegen_ssa.

[twilligon]

Should this issue be closed if master uses rustc_codegen_ssa?

[bjorn3]

Currently only a small part of rustc_codegen_ssa is used. Several other parts assume an LLVM-like backend is used. I am working on refactoring it to make it suitable for cranelift.