[scudo] Allow options to be defined at compile time

Summary:
Allow for options to be defined at compile time, like is already the case for
other sanitizers, via `SCUDO_DEFAULT_OPTIONS`.

Reviewers: alekseyshl, dberris

Reviewed By: alekseyshl, dberris

Subscribers: kubamracek, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D42980

llvm-svn: 324620