Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

Import new libcxxrt / libc++. This brings some bug fixes, including a potential race condition for static initialisers.

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.

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

Approved by: dim (mentor)

Update libcxxrt to remove the pthread dependency.

Also add the license from upstream to contrib.

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)