[scudo] Add -fsanitize=scudo option to Fuchsia

Apparently the -fsanitize flag hadn't been added for Scudo upstream yet.

Patch By: flowerhack

Reviewers: cryptoad, alekseyshl, mcgrathr, phosek

Reviewed By: mcgrathr, phosek

Differential Revision: https://reviews.llvm.org/D41413

llvm-svn: 321314