Fail if we try to crosscompile while PYTHON_SOABI is not set.

[AlexisTM]

[NOTE: This would be a workaround until python_cmake_module#7, which I couldn't make work locally]

When cross-compiling, we need the PYTHON_SOABI variable to be set.
The current method was using the system python interpreter instead of the target one, causing incorrect variable naming as in: #9 and ros2/pybind11_vendor#16

The current methodology is to provide to people cross-compiling with a sysroot (found using CMAKE_SYSROOT) with an user friendly error message to explain which variable should be set. (PYTHON_SOABI)

The possibly better way would be to execute the target python version with qemu-${ARCH}-static. This means we need to find the arch, check for qemu for this arch, find Python within the sysroot and execute it.
One argument against it is often a the DevOps creating the sysroot will have qemu, but it is not systematic for the developer side, thus creating possibly a "it works on my machine" clash against the DevOps.

Another consideration is, as we are using a sysroot, the python version will likely be fixed;

Sample output:

Starting >>> python_cmake_module
--- stderr: python_cmake_module                            
CMake Error at cmake/Modules/CheckCrossCompilingSoabi.cmake:31 (message):
  We detected you defined a sysroot thus crosscompiling.

  You have to define the PYTHON_SOABI variable in the toolchain file, which
  looks like cpython-39-x86_64-linux-gnu or cpython-39-aarch64-linux-gnu.

  To find the correct PYTHON_SOABI value, you can execute the following from
  the target:

  python3 -c "from sysconfig import
  get_config_var;print(get_config_var('SOABI'))"

  Run it with qemu with: (expecting aarch64 as your

  qemu-aarch64-static -L /opt/bosch/sysroot
  /opt/bosch/sysroot/usr/bin/python3 -c "from sysconfig import
  get_config_var;print(get_config_var('SOABI'))"

  The toolchain should look like:

  set(PYTHON_SOABI cpython-39-aarch64-linux-gnu)

Call Stack (most recent call first):
  CMakeLists.txt:17 (CheckCrossCompilingSoabi)


---
Failed   <<< python_cmake_module [0.23s, exited with code 1]

Summary: 36 packages finished [25.8s]
  1 package failed: python_cmake_module
  2 packages had stderr output: python_cmake_module uncrustify_vendor

[AlexisTM]

Fail early instead of on usage of this library (which is after compiling the rmw middleware).

[AlexisTM]

If we are cross-compiling, we need to bypass the PYTHON_MODULE_EXTENSION definition done by pybind11 too, or this fix will not have effect on libraries using pybind11 directly (aka rclpy).