MFC r303818, r303833, r303941, r304478, r304481, r304483, r304484, r304554,
r304555, r304556, r304557, r304558, r304559, r304561, r304563, r304564,
r304565, r304615, r304742, r304743, r304744, r304745, r304748, r304886,
r304991, r305928, r305938, r305987, r306185:

Bring CloudABI support back in sync with HEAD.

- Add support for running 32-bit executables on amd64, armv6 and i386.

- As these new architectures require the use of the vDSO, merge back
vDSO support for 64-bit executables running on amd64 and arm64 as
well. This has the advantage that support for vDSO-less execution
can be phased out when 11.0 becomes unsupported, as opposed to 11.x.

This change has been tested by running the cloudlibc unit tests on all
supported architectures, which seems to work fine.

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

Add support for CloudABI on ARM64.

It turns out that it is pretty easy to make CloudABI work on ARM64. We
essentially only need to copy over the sysvec from AMD64 and ensure that
we use ARM64 specific registers.

As there is an overlap between function argument and return registers,
we do need to extend cloudabi64_schedtail() to only set its values if
we're actually forking. Not when we're creating a new thread.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D3917

Fix mandoc warnings/errors.

MFC after: 1 week

Add a manual page for the cloudabi and cloudabi64 kernel modules.

CloudABI has two separate kernel modules: cloudabi and cloudabi64. The
first module contains all the pointer size independent code, whereas
cloudabi64 contains the actual 64-bits specific system calls and the ELF
loader.

Reviewed by: wblock
Obtained from: https://github.com/NuxiNL/freebsd
Differential Revision: https://reviews.freebsd.org/D3258