arm64: Add in_vhe() to find if the kernel is in VHE

Add a function to support devices that may need to know if the kernel
has enabled the Armv8.1 Virtulization Host Extensions (FEAT_VHE).
Some devices, e.g. the generic timer, will need to know, e.g. use a
different interrupt.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43973