Allow compiling usr.bin/find on Linux and Mac

When building FreeBSD the makefiles invoke find with various flags such as
-s that aren't supported in the native /usr/bin/find. To fix this I
build the FreeBSD version of find and use that when crossbuilding.

Inserting lots if #ifdefs in the code is rather ugly but I don't see a
better solution.

Reviewed By: brooks (mentor)
Approved By: jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D13306