[libFuzzer] Link libFuzzer's own interceptors when other compiler runtimes are not linked.

Summary: libFuzzer intercepts certain library functions such as memcmp/strcmp by defining weak hooks. Weak hooks, however, are called only when other runtimes such as ASan is linked. This patch defines libFuzzer's own interceptors, which is linked into the libFuzzer executable when other runtimes are not linked, i.e., when -fsanitize=fuzzer is given, but not others.

Reviewers: kcc, morehouse, hctim

Reviewed By: morehouse, hctim

Subscribers: krytarowski, mgorny, cfe-commits, #sanitizers

Tags: #clang, #sanitizers

Differential Revision: https://reviews.llvm.org/D83494