Remove $FreeBSD$: one-line sh pattern

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

Move all sources from the llvm project into contrib/llvm-project.

This uses the new layout of the upstream repository, which was recently
migrated to GitHub, and converted into a "monorepo". That is, most of
the earlier separate sub-projects with their own branches and tags were
consolidated into one top-level directory, and are now branched and
tagged together.

Updating the vendor area to match this layout is next.

libexecinfo: Integrate NetBSD test into FreeBSD

Sync up with NetBSD libexecinfo changes 2014-2019

Drop portions that are unlit or redundant with llvm-libunwind: builtin.c,
unwind.h, and unwind_arm_ehabi_stub.c.

This code should now work with -fPIE binaries, should we choose to build any
that way.

When backtrace() array is full, signal an error so the underlying
Itanium-style C++ exception handling library (llvm-libunwind) knows to stop
tracing instead of continuing. (It should stop on its own when it finishes
unwinding, so this is mostly an extra seatbelt against an infinite loop bug
in the unwinder.)

Use SRCTOP-relative paths to other directories instead of .CURDIR-relative ones

This simplifies pathing in make/displayed output

MFC after: 3 weeks
Sponsored by: Dell EMC Isilon

Update ^/vendor/NetBSD/tests/dist to a more recent snapshot

Pulled on "Thu Aug 11 18:01:19 PDT 2016"

First pass through library packaging.

Sponsored by: The FreeBSD Foundation

Convert libraries to use LIBADD
While here reduce a bit overlinking

Check in first src/tests snapshot from NetBSD anoncvs

Sources were obtained like so:

% export CVSROOT="anoncvs@anoncvs.NetBSD.org:/cvsroot"
% cvs -z9 co -D "09/30/2014 20:45" -P src/tests
% mv src/tests/* tests/dist/.

'*CVS*' has been added to svn:ignore to ease updating periodically from
upstream

Some line ending issues had to be resolved with test outputs and scripts
via dos2unix and by deleting the eol-style property set in usr.bin/sort

Discussed with: rpaulo
Sponsored by: EMC / Isilon Storage Division

Don't install private libexecinfo headers

libexecinfo compatibility with devel/libexecinfo port

1. Match shlib number
2. Add libelf dependency

Suggested by: bapt[1]

Add libexecinfo Makefile

Sponsored by: DARPA, AFRL