Porting SO3 on RISC-V and ARM64

SO3 is currently being ported on RISC-V architecture, and it will be ported on ARM64 in a near future. For these reasons, Makefiles and related KBuild files have been slightly adapted in order to support various architectures (arm32, arm64, riscv64). The master branch has been updated, but do not try to compile for arm64 yet ; it will not compile.
It does not change anything else with ARM32 based configurations.

The RISC-V integration has been partly achieved in the context of a diploma work (Nicolas Müller). It will be continued in the coming months, but volunteers are still welcome :blush: to go ahead as soon as possible.