bhyve: Build the HPET ACPI table on amd64 only

There is no HPET on arm64 and vm_get_hpet_capabilities() is not
implemented there. Move the vm_get_hpet_capabilities() call into
build_hpet(): I cannot see a downside, and doing so eliminates a global
variable and reduces the amount of code that needs to be conditionally
compiled. No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40990