Make cc_toolchain.cpu and cc_toolchain.compiler nonconfigurable

They should be used during toolchain selection (will happen in unknown commit). I
think it's an oversight that they are configurable now.

#6072

RELNOTES: None
PiperOrigin-RevId: 212791669

src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainRule.java

@@ -109,8 +109,8 @@ public RuleClass build(RuleClass.Builder builder, RuleDefinitionEnvironment env)
         .requiresConfigurationFragments(CppConfiguration.class, PlatformConfiguration.class)
         .advertiseProvider(TemplateVariableInfo.class)
         .add(attr("output_licenses", LICENSE))
-        .add(attr("cpu", STRING).mandatory())
-        .add(attr("compiler", STRING))
+        .add(attr("cpu", STRING).nonconfigurable("Used in configuration creation").mandatory())
+        .add(attr("compiler", STRING).nonconfigurable("Used in configuration creation"))
         .add(
             attr("all_files", LABEL)
                 .legacyAllowAnyFileType()