[fuzzer] Add attributes to LLVMFuzzerInitialize definition.

That prevents the linker from dead-stripping the function, as it is not called
directly, it is resolved in the runtime via dlsym().

Bug: chromium:754124, chromium:787723
Change-Id: I46a02ef01349f59b7ed944ce1483b7277e234a19
Reviewed-on: https://chromium-review.googlesource.com/833995
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50212}

test/fuzzer/fuzzer-support.cc

@@ -89,7 +89,11 @@ bool FuzzerSupport::PumpMessageLoop(
 
 }  // namespace v8_fuzzer
 
-extern "C" int LLVMFuzzerInitialize(int* argc, char*** argv) {
+// Explicitly specify some attributes to avoid issues with the linker dead-
+// stripping the following function on macOS, as it is not called directly
+// by fuzz target. LibFuzzer runtime uses dlsym() to resolve that function.
+extern "C" __attribute__((used)) __attribute__((visibility("default"))) int
+LLVMFuzzerInitialize(int* argc, char*** argv) {
   v8_fuzzer::FuzzerSupport::InitializeFuzzerSupport(argc, argv);
   return 0;
 }