Don't harcode -ldl test/sanitizer_common/TestCases

Summary:
The dl library does not exist on all system and in particular
this breaks build on NetBSD. Make it conditional and
enable only for Linux, following the approach from other
test suites in the same repository.

Reviewers: joerg, vitalybuka

Reviewed By: vitalybuka

Subscribers: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D52994

llvm-svn: 343987