[fuzzer] Declare LLVMFuzzerInitialize with attributes only if V8_OS_MACOSX.

R=ahaas@chromium.org, clemensh@chromium.org, mathias@chromium.org

Bug: chromium:754124, chromium:787723
Change-Id: I7eafee50a47ca0ad56a5458f1f232e3ed07c1cca
Reviewed-on: https://chromium-review.googlesource.com/834457Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50217}

test/fuzzer/fuzzer-support.cc

@@ -89,7 +89,14 @@ 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.
+#if V8_OS_MACOSX
+__attribute__((used)) __attribute__((visibility("default")))
+#endif  // V8_OS_MACOSX
+extern "C" int
+LLVMFuzzerInitialize(int* argc, char*** argv) {
   v8_fuzzer::FuzzerSupport::InitializeFuzzerSupport(argc, argv);
   return 0;
 }