arm64: Boot into VHE mode when able

When FEAT_VHE is present the HCR_EL2 E2H field will be implemented. Try
setting this and checking if it's still set to decide if we can boot
into VHE mode or not.

When it is implemented the kernel will boot into EL2 rather than EL1
it currently boots to. The parts of the kernel that need to know if
they are in EL1 or EL2 have been updated other than the CoreSight
driver as there doesn't appear to be any way to currently use it and
it will be updated soon with the new HWT framework.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46087