pmbr: Only load the first 545k rather than error out

It would be nice to have larger boot partitions for ESPs to live in one
day. It's trivial to carve out 5M 10M or 200M when provisioning, but
logistical issues may make it hard to do it after the fact. So only warn
when the partition is > 545k. If we ever grow the boot loader larger
than that, then it will be responsible for loading the rest anyway.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D42774

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Fix pmbr issues > 2TB

These issues have low impact because they require precise circumstances
to trigger one of them. The disk must be > 2 TiB in size and either:
- The primary GPT header is dammaged.
- The freebsd-boot partiton is located farther than the first 2 TiB of
the disc and one of its sectors takes place at a lba value that makes
the higher 32 bits of this very value change.
Errors and corrections folow:
- decl and incl don't affect CF, so replace with subl/addl $1
- repe uses %cx, so move size to it with movw
- moving a 64-bit value with %cx of 2 (should be 4) so addresses
> 2TB will work.

PR: 233180
Reviewed by: imp@ (applied patch using description in bug)
Differential Revision: https://reviews.freebsd.org/D31100

stand/i386: rename .s to .S to use Clang IAS

As part of the migration away from obsolete binutils we want to retire
GNU as. Most assembly files used on amd64 have a .S extension and
(via rules in share/mk/bsd.suffixes.mk) are assembled with Clang's
Integrated Assembler (IAS). Rename files in stand/i386 to .S to use
the integrated assembler.

Clang's IAS supports the defsym option (via -Wa,) but only with one
dash, not two. As both -defsym and --defsym are accepted by GNU as,
use the former.

PR: 233611
Reviewed by: tsoome
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18369