MFC r325834,r325997,326502: Move sys/boot to stand/

This is effectively a direct commit to stable/11, due to differences between
stable/11 and head. Changes to DTS in sys/boot/fdt/dts were often
accompanied by kernel changes. Many of these were also risc-v updates that
likely had many more dependencies to MFC.

Because of this, sys/boot/fdt/dts remains as-is while everything else in
sys/boot relocates to stand/.

r325834: Move sys/boot to stand. Fix all references to new location

r325997: Remove empty directories.

r326502: Document the sys/boot -> stand move in hier.7 and the top-level README.

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

sys/boot: spelling fixes in comments.

No functional change.

Enable warnings in EFI boot code

Set WARNS if not set for EFI boot code and fix the issues highlighted by
setting it.

Most components are set to WARNS level 6 with few being left at lower
levels due to the amount of changes needed to fix at higher levels.

Error types fixed:
* Missing / invalid casts
* Missing inner structs
* Unused vars
* Missing static for internal only funcs
* Missing prototypes
* Alignment changes
* Use of uninitialised vars
* Unknown pragma (intrinsic)
* Missing types etc due to missing includes
* printf formatting types

Reviewed by: emaste (in part)
MFC after: 2 weeks
X-MFC-With: r293268
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D4839

Fix _MSC_EXTENSIONS checks

Use #ifdef instead of #if checks to prevent warnings generated by checks
to be enabled shortly.

MFC after: 2 weeks
Sponsored by: Multiplay

Revert accidental whitespace changes included with r292623

Support a.out format in nlist only on i386

i386 is the only current FreeBSD architecture that ever used a.out
format.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D4687

Add support for arm64 to loader.efi and boot1.efi

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation