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

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

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 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)