[HEADS UP] enabling LLDB debugger by default on amd64

The in-tree snapshot of LLDB is at a point where it's usable and
suitable for wider testing on amd64, and so I intend to enable it by
default in the near future.

Further information on the FreeBSD port of LLDB is on the wiki, at
https://wiki.freebsd.org/lldb

On my desktop LLDB added about 5 minutes to a buildworld and 80MB to
objdir (over a baseline of about an hour and 1.8GB).  If you wish to
avoid building it, you can add 'WITHOUT_LLDB=' to src.conf.

-Ed