Port native tools to use KF5_HOST_TOOLING

The previous hack of just copying the native tools into the target sysroot
has several limitations:
- it no longer works with relative RPATHs being used
- it fails in the case of kpackage where we need the same tools for host
  and target

classes/cmake_kf5.bbclass

@@ -1,10 +1,16 @@
 inherit cmake_qt5
 
-EXTRA_OECMAKE += " \
+EXTRA_OECMAKE_class-native += " \
     -DOE_KF5_PATH_HOST_ROOT=${STAGING_DIR_HOST} \
     -DBUILD_TESTING=OFF \
 "
 
+EXTRA_OECMAKE_class-target += " \
+    -DOE_KF5_PATH_HOST_ROOT=${STAGING_DIR_HOST} \
+    -DBUILD_TESTING=OFF \
+    -DKF5_HOST_TOOLING=${STAGING_DIR_NATIVE}/${libdir}/cmake \
+"
+
 DEPENDS += "extra-cmake-modules qttools-native"
 
 # don't bother with translations for host tools