[scudo] Fix standalone compiler-rt test breakage

Summary:
Apparently "test standalone compiler-rt" still requires -ldl and -lrt for
Scudo even with --gc-sections. I am not entirely sure why, so if anybody has
some input, feel free to chime in.

In the meantime, add again those two to fix the test.

Reviewers: kcc, alekseyshl

Reviewed By: kcc

Subscribers: Hahnfeld, dberris, llvm-commits

Differential Revision: https://reviews.llvm.org/D29527

llvm-svn: 294199