Auto merge of #145955 - bjorn3:lto_refactors4, r=nnethercote

Rework how the codegen coordinator code handles the allocator shim

Continuing from rust-lang/rust#144503 this centralizes most handling of the allocator shim to a single 4 line block in the codegen coordinator. The allocator shim is small enough that making it go through the main codegen loop and spawning a worker thread for it is wasted effort.

Author: bors