Auto merge of #83637 - bjorn3:sync_cg_clif-2021-03-29, r=bjorn3

Auto merge of #83637 - bjorn3:sync_cg_clif-2021-03-29, r=bjorn3

Sync rustc_codegen_cranelift

The main highlight of this sync is support for cross-compiling to Windows using MinGW. Native compilation with MinGW would also work I think, but using the MSVC toolchain is not yet supported as PE TLS is not yet implemented. Another nice improvement is that crate metadata is now loaded using mmap instead of by reading files. This improves compilation time a bit.

r? `@ghost`

`@rustbot` label +A-codegen +A-cranelift +T-compiler