MFC r316732, r316756: Enable Privileged Access Never on arm64.

r316732:
Use the unprivileged variant of the load and store instructions most
places possible in the kernel. This forces these functions to fail if
userspace is unable to access a given memory location, even if it is in
the user memory range.

This will simplify adding Privileged Access Never support later.

r316756:
In ARMv8.1 ARM has added a process state bit to disable access to userspace
from the kernel. Make use of this to restrict accessing userspace to just
the functions that explicitly handle crossing the user kernel boundary.

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

Mark functions as such. This means we call them directly rather than have
the dynamic linker copy them, but not relocate them at the new location.
This allows us to run sqlite3 without it crashing.

Sponsored by: ABT Systems Ltd

Add the start of the arm64 machine headers. This is the subset needed to
start getting userland libraries building.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation