[libFuzzer] Add preprocessor flag to explicitly enable weak symbols on Mac.

[Dor1s]

Summary

Default implementation for Mac (i.e. FuzzerExtFunctionsDlsym.cpp) does
not work when LLVMFuzzer* interface functions are defined in different
translation units and compiled with "-fvisibility=hidden" flag, as the linker
removes those as a dead code.

That is an alternative solution if we cannot adjust Chromium build config:
https://chromium-review.googlesource.com/c/chromium/src/+/812108

lib/fuzzer/FuzzerExtFunctionsDlsym.cpp

 //===- FuzzerExtFunctionsDlsym.cpp - Interface to external functions ------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 // Implementation for operating systems that support dlsym(). We only use it on
 // Apple platforms for now. We don't use this approach on Linux because it
 // requires that clients of LibFuzzer pass ``--export-dynamic`` to the linker.
 // That is a complication we don't wish to expose to clients right now.
 //===----------------------------------------------------------------------===//
 #include "FuzzerDefs.h"
-#if LIBFUZZER_APPLE
+#if LIBFUZZER_APPLE && !LIBFUZZER_WITH_UNDEFINED_WEAK_SYMBOLS
 
 #include "FuzzerExtFunctions.h"
 #include "FuzzerIO.h"
 #include <dlfcn.h>
 
 using namespace fuzzer;
 
 template <typename T>

lib/fuzzer/FuzzerExtFunctionsWeak.cpp

 //===- FuzzerExtFunctionsWeak.cpp - Interface to external functions -------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 // Implementation for Linux. This relies on the linker's support for weak
 // symbols. We don't use this approach on Apple platforms because it requires
 // clients of LibFuzzer to pass ``-U _<symbol_name>`` to the linker to allow
 // weak symbols to be undefined. That is a complication we don't want to expose
-// to clients right now.
+// to clients right now. However, if someone prefers that way, they need to
+// explicitly define LIBFUZZER_WITH_UNDEFINED_WEAK_SYMBOLS.
 //===----------------------------------------------------------------------===//
 #include "FuzzerDefs.h"
-#if LIBFUZZER_LINUX || LIBFUZZER_NETBSD
+#if LIBFUZZER_LINUX || LIBFUZZER_NETBSD || LIBFUZZER_WITH_UNDEFINED_WEAK_SYMBOLS
 
 #include "FuzzerExtFunctions.h"
 #include "FuzzerIO.h"
 
 extern "C" {
 // Declare these symbols as weak to allow them to be optionally defined.
 #define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
   __attribute__((weak)) RETURN_TYPE NAME FUNC_SIG