Use the platform headers in the pass-2 and canadian gcc stages

scripts/build/cc/gcc.sh

@@ -350,12 +350,11 @@ do_gcc_core_backend() {
             ;;
     esac
 
-    # This is only needed when building libstdc++ in a canadian environment with
-    # this function being used for final step (i.e., when building for bare metal).
-    if [ "${build_step}" = "gcc_build" ]; then
+    case "${build_step}" in
+        core2|gcc_build)
             CT_DoLog DEBUG "Copying headers to install area of core C compiler"
             CT_DoExecLog ALL cp -a "${CT_HEADERS_DIR}" "${prefix}/${CT_TARGET}/include"
-    fi
+    esac
 
     for tmp in ARCH ABI CPU TUNE FPU FLOAT ENDIAN; do
         eval tmp="\${CT_ARCH_WITH_${tmp}}"