[ahrs] Add Microstrain GX3 AHRS subystem

The AP stays in kill mode until it gets valid data from AHRS (using ahrs_aligner)
[ahrs] clean ahrs_gx3

- cleanup indentation, remove trailing whitespaces
- removed unused USE_GX3 flag
- move makefile to shared, as in principle it should work for fixedwings as well
- make ImuEvent a cleaner static inline function
[conf] add radio file for Turnigy 9XR from Michal