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.

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

Allow EFI and ACPI to be included together. When ACPI is included
first, EFI will use its definitions for {,U}INT{8,16,32,64} and
BOOLEAN. When EFI is included first, define ACPI_USE_SYSTEM_INTTYPES
to tell ACPI that these are already defined.

Differential Revision: https://reviews.freebsd.org/D1905

Sync the EFI headers with version 1.10.14.62 of the Intel sample EFI
implementation. This re-introduces C99 style comments that previously
were replaced by original C comments.

Don't try to use 'typedef struct foo' if just 'struct foo' makes more sense
and works on all compilers. This also removes the need for
__CC_SUPPORTS_FORWARD_REFERENCE_CONSTRUCT in <sys/cdefs.h>.

OK'ed by: marcel, dfr

An almost mechanical sweep to replace C++ style comments with C
style comments. This is not an attempt to conform to style(9).
Such has lower priority.

First approximation of an ia64 EFI loader. Not functional.