Install unwind.h into /usr/include.

[jhb]

Summary

Install headers from LLVM's libunwind in place of the headers from
libcxxrt and allow C applications to use the library.

As part of this, remove include/unwind.h and switch libthr over to
using the installed unwind.h.

[jhb]

This is just a small superset of D34005 in that it also deals with include/unwind.h.

[emaste]

Note exp-run from D34005 is in https://bugs.freebsd.org/261412

[emaste]

LGTM pending the exp-run

[dim]

I'm going to commit this, with two minor cosmetic changes:

• Fill in actual date in ObsoleteFiles.inc
• Sort INCS list in lib/libgcc_eh/Makefile

[pkubaj]

Could you also MFC this to stable/13?

[pkubaj]

Nevermind, there is MFC note in the commit.

[dim]

In D34065#776434, @pkubaj wrote:

Nevermind, there is MFC note in the commit.

Yeah, though this should bake a little longer than 3 days. I also wonder who is going to get the MFC reminder mail, the author (@jhb) or the committer (me) :)