Make src.opts.mk the source of truth for whether or not to build hyperv

In particular, according to several Makefiles it can only be built/run on
amd64 and i386/i386. It isn't supported on i386/pc98
Add an additional src.conf doc snippet for WITH_HYPERV
Make building/installing bhyve optional via MK_BHYVE on amd64

Disable building/installing bhyve from src.opts.mk on unsupported
architectures (!amd64)