MFC r258892: lldb: Threaded inferior support for FreeBSD

This is in the process of being submitted to the upstream LLDB
repository. The thread list functionality is modelled in part on
GDBRemoteCommunicationClient.

LLDB bug pr16696 and code review D2267

Sponsored by: DARPA, AFRL

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