Add a dlltool attribute to rust_toolchain (#4330)

`dlltool_path_from_linker_path` derives `dlltool` from the
cc_toolchain's linker path, which assumes a MinGW binutils layout.
Toolchains that name it differently (`llvm-dlltool` in hermetic-llvm) or
don't ship it next to the linker can't build non-MSVC Windows targets.

This adds an optional `dlltool` attribute to `rust_toolchain`, also
exposed on the `rust.toolchain` and `rust.repository_set` extension
tags. When set, it's passed with `--codegen=dlltool=` and added to the
Rustc action inputs. When unset, the current behavior is kept.

Fixes #4315