[clang-fuzzer] Allow linking with any fuzzing engine.

Summary:
Makes clang-[proto-]fuzzer compatible with flags specified by OSS-Fuzz.

https://llvm.org/pr34314

Reviewers: vitalybuka, kcc

Reviewed By: kcc

Subscribers: cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D38812

llvm-svn: 315486