arch-riscv: report that we don't have debugging support.

arch-riscv: report that we don't have debugging support.

According to the debugging spec (page 47), a debugger can test which
triggers are enabled by writing 0 to TSELECT and reading it back. If a
different value is read, the trigger is not supported.

Therefore, we currently always set a different value to indicate that
we do not support any triggers.

Change-Id: If222e913c4517adb2da4f6f0ffeedb4e4808a586
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25659
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>