arm64: Connect bhyve and libvmmapi to the build

Reviewed by: corvink, andrew, jhb, emaste
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D41742

ofwdump: install dependent on MK_FDT

The utility depends on the presence of the openfirm(4) pseudo-device,
which is gated by the FDT kernel option. The MK_FDT knob is correctly
set to "yes" for FDT/OFW-enabled platforms (powerpc* included); use it
to install the utility and eliminate the arch-specific Makefile logic.

No functional change intended.

Reviewed by: emaste, pkubaj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41977

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Fix the name of Makefile.aarch64

Makefile.arm64 relies on being included via Makefile.${MACHINE}. This
only works when arm64 is the only aarch64 architecture. Switch to
Makefile.aarch64 so downstream CheriBSD can use the file when building
for Morello which uses a differen ${MACHINE} value.

Sponsored by: Innovate UK