python: changed to install the pgen we need at cross stage and which runs on the build host

We inject the `pgen` built at toolchain stage as `python-pgen` which we use
at cross stage (see: use-hostpgen.patch.cross).

python/python/python.conf

@@ -52,6 +52,12 @@ fix_python_for_64bit() {
 
 [[ $libdir = *lib64 ]] && hook_add preconf 9 "fix_python_for_64bit"
 
+# toolchain tweaks
+if atstage toolchain; then
+	# we need pgen at cross stage that runs on the build host
+	hook_add postmake 5 'cp Parser/pgen $root$bindir/python-pgen'
+fi
+
 # cross-compiling tweaks
 if atstage cross; then
 	var_append configcache ' ' "ac_cv_buggy_getaddrinfo=no"