Call out to binutils' dlltool for raw-dylib on windows-gnu platforms.

compiler/rustc_session/src/options.rs

@@ -1073,6 +1073,8 @@ options! {
     dep_tasks: bool = (false, parse_bool, [UNTRACKED],
         "print tasks that execute and the color their dep node gets (requires debug build) \
         (default: no)"),
+    dlltool: Option<PathBuf> = (None, parse_opt_pathbuf, [UNTRACKED],
+        "import library generation tool (windows-gnu only)"),
     dont_buffer_diagnostics: bool = (false, parse_bool, [UNTRACKED],
         "emit diagnostics rather than buffering (breaks NLL error downgrading, sorting) \
         (default: no)"),