Enable GetRandom for Fuchsia sanitizer.

Summary: Adds a true implementation of GetRandom, to be used by scudo_utils.h.

Reviewers: mcgrathr, phosek, kcc, vitalybuka, cryptoad

Reviewed By: mcgrathr

Subscribers: kubamracek

Differential Revision: https://reviews.llvm.org/D37218

llvm-svn: 312046