[WIP] add clang arch for RISCV

[noferini]

I got a maching at CNAF to test the build on RISC-V
This option is needed to compile clang with such an arch
Still in the process of building, I'll remove WIP as soon as I check other packages don't need any further action

[ktf]

As discussed privately, you probably want riscvarchive/riscv-gcc@62da3c0 applied to GCC-Toolchain to avoid the spurious dependency on libatomic.