[scudo] Fix standlone build -lrt requirement

Summary:
The split in D39461 introduced separate C++ flags, but `cxx_flags` needs `-lrt` as well for the standalone build.


Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39497

llvm-svn: 317103