Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r245304 (by theraven):

Merge new version of libcxxrt. This brings in three fixes:

- Don't treat pointers to members as pointers in catch blocks
(they're usually fat pointers).

- Correctly catch foreign exceptions in catchalls.

- Ensure that a happens-before relationship is established when
setting terminate handlers in one thread and calling them in
another.

MFC r245327 (by theraven):

Fix libcxxrt / libc++ build with the clang in head.

Pointy hat to: theraven

MFC r245745 (by theraven):

Import new fix from libcxxrt. This fixes the case where you attempt
to rethrow an exception when you haven't caught one. This is largely
a cosmetic fix, as (unless you have a very unusual terminate handler
installed) it will print a nice error and then abort, rather than
just aborting.

MFC r246462 (by theraven):

Fix a copy-and-paste error in libcxxrt.

MFC r249993:

Merge libcxxrt c812a07cd2f95c1403baf0bbe0366e7618d1d6d3:

* Don't call the _fast version of the TLS accessor in terminate() or
unexpected().
1) TLS may not have been set up yet.
2) When we're in one of these functions, Really Bad Stuff has
happened and potentially saving a few cycles really isn't
important.
* Merge in fixes from FreeBSD trunk to make atomics work with recent
clang.

Merged libcxxrt and libc++. Now available for testing on 9-stable with
-stdlib=libc++. Changes to libstdc++ not yet merged, so it is not yet possible
to mix libstdc++ and libc++ in the same program.

Merged revisions: 226702,226785,227006,227755,227983,227987,228531,228630,228761,229067,230127,232950,233098,234715-234716,234772

Import new version of libcxxrt. Now works correctly with libobjc2 to implement
the unified exception model for Objective-C++.

Approved by: dim (mentor)

Import new versions of libcxxrt and libc++.
Please tests any C++ code you care about with -stdlib=libc++!

Approved by: dim (mentor)

Import libc++ / libcxxrt into base. Not build by default yet (use
MK_LIBCPLUSPLUS=yes to enable). This is a work-in-progress. It works for
me, but is not guaranteed to work for anyone else and may eat your dog.

To build C++ using libc++, add -stdlib=libc++ to your CXX and LD flags.

Bug reports welcome, bug fixes even more welcome...

Approved by: dim (mentor)

libcxxrt version snapshot created.

Approved by: dim (mentor)

Import libcxxrt / libc++ into a vendor branch.

Approved by: dim (mentor)