PYTHON_SOABI returns a host version of SOABI while cross-compiling

Bug report

Required Info:

• Operating System:
  • host: Ubuntu 18.04
  • target: VxWorks 7
• Installation type:
  • source compilation
• Version or commit hash:
  • dashing release
• DDS implementation:
  • N/A
• Client library (if applicable):
  • N/A

Steps to reproduce issue

While cross-compiling the ROS2 dashing release for VxWorks and encounter the following problem
FindPythonExtra.cmake gives back a host PYTHON_SOABI instead of a target one.
It runs get_config_var('SOABI') using native python
"from sysconfig import get_config_var"
"print(get_config_var('SOABI'))"

After that all python ROS2 modules are built with the host extension and not loadable on the target system.
build: _rclpy.cpython-38-x86_64-linux-gnu.so
expected: _rclpy.cpython-38-vxworks.so

Expected behavior

It should give back a target SOABI, not the host one.
Probably a bit more sophisticated check shall be implemented, e.g. run a target version of
python3-config --extension-suffix

Actual behavior

always returns the host python SOABI version

Additional information

It is possible to define -DPYTHON_SOABI="my SOABI" to workaround it.