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

Add LLDB bmake infrastructure

This connects LLDB to the build, but it is disabled by default. Add
WITH_LLDB= to src.conf to build it.

Note that LLDB requires a C++11 compiler so is disabled on platforms
using GCC.

Approved by: re (gjb)
Sponsored by: DARPA, AFRL