Auto merge of #26098 - russellmcc:add-ar-tool-flexible-target-spec, r=alexcrichton

Looks like this was missed from af56e2efde.

This will help with defining cross-compile workflows for rust.

src/librustc_back/target/mod.rs

@@ -272,6 +272,7 @@ impl Target {
         }
 
         key!(cpu);
+        key!(ar);
         key!(linker);
         key!(relocation_model);
         key!(code_model);