I finally managed to make it work on my Raspberry PÏ 3B.
The Archlinux Arm Aarch64 (ARMv8 64 bits) does not work because, the library libNRJavaSerial.so is not 64 bits compliant (at this time).
So I had to stick with Archlinux ARM 32bits (ARMv7 version).
Recommended Java version for openhab is Java 8. I couldn’t make Zulu to work, so I installed the Oracle JDK. The package jdk-arm provides it and is available in AUR.
And openhab2 package is also available in AUR. However this last does not add the openhab user to the uucp group which is mandatory to access serial devices. I already added a comment in the AUR package about this.
Both RFXcom and MySensors’ gateway are properly working.
I hope this can be useful for someone 