registerArch advertises support for vector registers for riscv backend but register allocator doesn't support it (= panic)

The registerArch function is used to indicate the register architecture.

For riscv64 it was advertising Separate, which includes vector register support. However the linear register allocator doesn't support vector registers which led to a panic

riscv64-linux-gnu-ghc: sorry! (unimplemented feature or known bug)
  GHC version 9.11.20240927:
    Linear.RV64.getFreeRegs: vector registers are not supported

The panic arises from clobberRegs, which calls getFreeRegs.

In order to unbreak things for now, I am going to change it to NoVectors.