MFC r333436: only launch getty if underlying device exists

Note: pc98 isn't run under VM, so I didn't do a direct commit to it.

Approved by: re@ (gjb@)

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Update the arm64 ttys file to enable the correct uart based on which device
the console is attached to. This makes this file identical to the 32-bit
arm version of this file.

Obtained from: EuroBSDCon devsummit
Sponsored by: ABT Systems Ltd

Start to import support for the AArch64 architecture from ARM. This change
only adds support for kernel-toolchain, however it is expected further
changes to add kernel and userland support will be committed as they are
reviewed.

As our copy of binutils is too old the devel/aarch64-binutils port needs
to be installed to pull in a linker.

To build either TARGET needs to be set to arm64, or TARGET_ARCH set to
aarch64. The latter is set so uname -p will return aarch64 as existing
third party software expects this.

Differential Revision: https://reviews.freebsd.org/D2005
Relnotes: Yes
Sponsored by: The FreeBSD Foundation