Add build compiler/rustc_codegen_gcc as an alias for CodegenBackend

These paths (`_cranelift` and `_gcc`) are somewhat misleading, since they
actually tell bootstrap to build *all* codegen backends. But this seems like
a useful improvement in the meantime.

Author: jyn514

src/bootstrap/compile.rs

@@ -795,7 +795,7 @@ impl Step for CodegenBackend {
     const DEFAULT: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        run.path("compiler/rustc_codegen_cranelift")
+        run.paths(&["compiler/rustc_codegen_cranelift", "compiler/rustc_codegen_gcc"])
     }
 
     fn make_run(run: RunConfig<'_>) {