Replace the implementation of DTrace's RAND subroutine for generating
low-quality random numbers with a modern implementation (xoroshiro128+)
that is capable of generating better quality randomness without compromising performance.

Submitted by: Graeme Jenkinson
Reviewed by: markj
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D9051