leak sanitizer support

Hello hackers,

It's almost new year and I'm cleaning out the clutters on my hard drives,
I came across some local mods that I feel someone else might also find useful.
One of them is the leak sanitizer support, would anyone be interested?
I've uploaded the diff to https://people.freebsd.org/~luoqi/lsan/. It's against
-stable/13, but most likely would apply cleanly to -current.

Happy new year!
-luoqi