new port: java/lightweight-java-profiler

A port of the lightweight-java-profiler (ljp). ljp acts as a bare-bones agentlib to produce profiling stacks that can be digested, e.g., by Brendan Gregg's flamegraph scripts.

Reviewed by: mat, swills (mentor)
Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D10194