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.

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<'_>) {