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

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

Import a new libcxxrt. This fixes some potential crashing in the demangler.

Approved by: re (gjb)
MFC after: 1 week

Remove __attribute__((__aligned__)) that generates incorrect alignment on compilers that know about SSE. Code left over from old header that thought that 64 bits was the maximum alignment that anything would ever need...

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 the new version of libcxxrt, which grew an extra ARM.

Approved by: dim (mentor)