arch-riscv: let FPU instructions fault if status.FS = off.

arch-riscv: let FPU instructions fault if status.FS = off.

These checks are required for some tests in the RISC-V test suite.
However, actually we also need to set the INITIAL/CLEAN/DIRTY flags
accordingly, which is not done yet.

Change-Id: If5d6ac22069b51a57b6353cd6d45b77ee51a4d55
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25657
Tested-by: kokoro <noreply+kokoro@google.com>
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>