Hypervisor support (#549)

* Add most hypervisor support

* Don't check in PDF changes.

* Add vu/vs bits to various triggers
Clarify that registers are "accessible" in specified modes, not just "writable"
Prohibit hcontext alias when the H extension isn't supported since this is in
  the CSR address space that's normally just for hypervisor.

* Clarify that the backwards-compatible mscontext is optional.

* Update core_debug.tex

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>

* Fix curly braces

* Fix textra64.mhselect
Remove comments about future H mode support

* scontext is accessible from VS mode.
Clarify in various places that enables are for "S/HS" instead of just "S" mode.

* Use decimal values in field descriptions to match the rest of the spec.
VMID is in hgatp, not satp.

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>