message: Implement RFC 2951: Native link modifiers
  This commit implements both the native linking modifiers infrastructure
  as well as an initial attempt at the individual modifiers from the RFC.
  It also introduces a feature flag for the general syntax along with
  individual feature flags for each modifier.
diff: @@ -85,8 +85,8 @@ fn add_file(&mut self, file: &Path) {
  ));
  }
  - fn add_native_library(&mut self, name: rustc_span::symbol::Symbol) {
  - let location = find_library(name, &self.lib_search_paths, self.sess);
  + fn add_native_library(&mut self, name: rustc_span::symbol::Symbol, verbatim: bool) {
  + let location = find_library(name, verbatim, &self.lib_search_paths, self.sess);
  self.add_archive(location.clone(), |_| false).unwrap_or_else(|e| {
  panic!("failed to add native library {}: {}", location.to_string_lossy(), e);
  });
  