MFC r316734, r316761: Support Execute-Never bit in the arm64 pmap.

r316734:
Start to use the User and Privileged execute-never bits in the arm64
pagetables. This sets both bits when entering an address we know shouldn't
be executed.

I expect we could mark all userspace pages as Privileged execute-never to
ensure the kernel doesn't branch to one of these addresses.

While here add the ARMv8.1 upper attributes.

r316761:
Set the arm64 Execute-never bits in more places.

We need to set the Execute-never bits when mapping device memory as the
hardware may perform speculative instruction fetches.

Set the Privileged Execute-ever bit on userspace memory to stop the kernel
if it is tricked into executing it.

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

Fix I/O coherence issues on ThunderX when SMP is disabled

To maintain coherence between cache and DMA memory appropriate
shareability flags need to be set in the PTE regardless of SMP
option.

Reviewed by: wma
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D6231

Add support for 4 level pagetables. The userland address space has been
increased to 256TiB. The kernel address space can also be increased to be
the same size, but this will be performed in a later change.

To help work with an extra level of page tables two new functions have
been added, one to file the lowest level table entry, and one to find the
block/page level. Both of these find the entry for a given pmap and virtual
address.

This has been tested with a combination of buildworld, stress2 tests, and
by using sort to consume a large amount of memory by sorting /dev/zero. No
new issues are known to be present from this change.

Reviewed by: kib
Obtained from: ABT Systems Ltd
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5720

Remove the invalid L0_BLOCK definition. ARMv8 doesn't support block
translation in the level 0 descriptor.

Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation

Set memory to be inner-sharable. This isn't needed on device memory as the
MMU will ignore the attribute there, howeverit simplifies to code to alwas
set it.

Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation

Add the definition of the shareable bits in the pagetables

Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation

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