loader: add target for dirdeps build

Update dependencies for the loader variations used for each
architecture.

Reviewed by: sjg
Differential Revision: https://reviews.freebsd.org/D39741

Remove $FreeBSD$: one-line sh pattern

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

Allow site.*.mk to augment local.*.mk and src.*.mk

Add some extra customization points so that FreeBSD build
can be adapted to local requirements.
We use these to minimize changes to share/mk

Reviewed by: stevek
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D37617

share/mk heimdal: Remove unnecessary trailing backslash

Remove the trailing backslash from the last list item. This is a NOP and
never bothered anything because the next line was a blank line. Remove
it for correctness sake.

MFC after: 3 days

retire BSD_CRTBEGIN option

BSD crt is currently used on all architectures (other than sparc64).
Remove the option and use BSD crt everywhere as part of the GCC 4.2.1
retirement plan.

https://lists.freebsd.org/pipermail/freebsd-arch/2020-January/019823.html

PR: 239851
Reviewed by: andrew, brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23122

Update dirdeps.mk and gendirdeps.mk

The env space consumed by exporting all libc's .meta files
left little room for command line,
so unexport when done.

Update dirdeps.mk to latest and add
dirdeps-targets.mk to simplify/update targets/Makefile

Makefile changes to go with Makefile.depend changes in D22494

Reviewed by: bdrewery
MFC after: 1 week
Sponsored by: Juniper Networks
Differential Revision: https://reviews.freebsd.org/D22495