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.

compiler/rustc_span/src/symbol.rs

@@ -748,6 +748,7 @@ symbols! {
         minnumf64,
         mips_target_feature,
         misc,
+        modifiers,
         module,
         module_path,
         more_struct_aliases,
@@ -763,6 +764,11 @@ symbols! {
         naked,
         naked_functions,
         name,
+        native_link_modifiers,
+        native_link_modifiers_as_needed,
+        native_link_modifiers_bundle,
+        native_link_modifiers_verbatim,
+        native_link_modifiers_whole_archive,
         ne,
         nearbyintf32,
         nearbyintf64,