Greatly simplify the macros for flags that affect target modifiers

By using the unstable `${ignore(..)}` metavar to help perform conditional
expansion for a subset of list items, we can eliminate several complex helper
macros.

Author: Zalathar

compiler/rustc_session/src/lib.rs

@@ -5,10 +5,8 @@
 #![feature(default_field_values)]
 #![feature(iter_intersperse)]
 #![feature(macro_derive)]
+#![feature(macro_metavar_expr)]
 #![feature(rustc_attrs)]
-// To generate CodegenOptionsTargetModifiers and UnstableOptionsTargetModifiers enums
-// with macro_rules, it is necessary to use recursive mechanic ("Incremental TT Munchers").
-#![recursion_limit = "256"]
 // tidy-alphabetical-end
 
 pub mod errors;