[cmake] Always run native build standalone

Summary:
This ensure 2 goals:

• The cross compilation and regular build take the same codepath, which ensure it is more used and therefore will stay robust.
• The native executables can have different build options that generated ones.

Test Plan:

• Do a native compilation using cmake
• Cross compile for ARM

Both are green.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5001