Rollup merge of #122810 - nnethercote:rm-target_override, r=WaffleLapkin

Remove `target_override`

Because the "target can override the backend" and "backend can override the target" situation is a mess. Details in the individual commits.

r? `@WaffleLapkin`

compiler/rustc_target/src/spec/mod.rs

@@ -2092,6 +2092,9 @@ pub struct TargetOptions {
     /// compiling `rustc` will be used instead (or llvm if it is not set).
     ///
     /// N.B. when *using* the compiler, backend can always be overridden with `-Zcodegen-backend`.
+    ///
+    /// This was added by WaffleLapkin in #116793. The motivation is a rustc fork that requires a
+    /// custom codegen backend for a particular target.
     pub default_codegen_backend: Option<StaticCow<str>>,
 
     /// Whether to generate trap instructions in places where optimization would