Fix stack_trace_compressor builds for Clang < 6.0
Clang 4.* doesn't supply -fsanitize=fuzzer, and Clang 5.* doesn't supply
-fsanitize=fuzzer-no-link. Generally, in LLVM, fuzz targets are added through
the add_llvm_fuzzer build rule, which can't be used in compiler-rt (as it has
to be able to be standalone built).
Instead of adding tooling to add a dummy main (which kind of defeats the
purpose of these fuzz targets), we instead build the fuzz target only when the
Clang version is >= 6.*.
Subscribers: mgorny, #sanitizers, llvm-commits
Tags: #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D66682