Initialize x registers in p, pm, pt in addition to v

[pdonahue-ventana]

https://github.com/riscv/riscv-test-env/commit/e0339fc0775e0e8d478462ce3ed055ccde33f76a avoided the whack-a-mole thing and helped debugging (and, in a few restricted scenarios, it avoided x-prop issues). This extends the same thing to p, pm, and pt for the same reasons.

[aswaterman]

Yep, might as well do the same thing here.