Auto merge of #72049 - mati865:mingw-lld, r=petrochenkov

MinGW: enable dllexport/dllimport

Fixes (only when using LLD) https://github.com/rust-lang/rust/issues/50176
Fixes https://github.com/rust-lang/rust/issues/72319

This makes `windows-gnu` on pair with `windows-msvc` when it comes to symbol exporting.
For MinGW it means both good things like correctly working dllimport/dllexport, ability to link with LLD and bad things like https://github.com/rust-lang/rust/issues/27438.

Not sure but maybe this should land behind unstable compiler option (`-Z`) or environment variable?

diff --cc src/librustc_codegen_llvm/callee.rs
Simple merge

diff --cc src/librustc_codegen_llvm/consts.rs
Simple merge

diff --cc src/librustc_codegen_llvm/context.rs
Simple merge

diff --cc src/librustc_codegen_ssa/back/linker.rs
Simple merge

diff --cc src/librustc_session/session.rs
Simple merge

diff --cc src/test/run-make-fulldeps/tools.mk
Simple merge