soc/cores/cpu/naxriscv fix arch definition and small adjust

[trabucayre]

With recent gcc software fails with:

litex/litex/soc/cores/cpu/naxriscv/system.h:12: Error: unrecognized opcode `fence.i', extension `zifencei' required

fence.i seems have been moved out of i ISA

rvXXima arch must be replaced by rvXXi2p0_ma

[trabucayre]

The second commit is to avoid some issue when target build is done into a python script. Before the git clone os.chdir is called. But after this step current directory remains pythondata-cpu-naxriscv and caller script fails if some task must be done after without using absolute path.

[enjoy-digital]

Thanks @trabucayre, that's merged.