Merge #18008: test: only declare a main() when fuzzing with AFL

b35567f test: only declare a main() when fuzzing with AFL (fanquake)

Pull request description:

  This fixes fuzzing using [libFuzzer](https://llvm.org/docs/LibFuzzer.html) on macOS, which caused a few issues during the recent review club. macOS users could only fuzz using afl, or inside a VM.

  It seems that the `__attribute__((weak))` marking is not quite enough to properly mark `main()` as weak on macOS. See Apples docs on [Frameworks and Weak Linking](https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html#//apple_ref/doc/uid/20002378-107262-CJBJAEID).

  Have tested fuzzing using libFuzzer and AFL with this patch.

ACKs for top commit:
  MarcoFalke:
    ACK b35567f
  fjahr:
    ACK b35567f

Tree-SHA512: b881fdd98c7e1587fcf44debd31f5e7a52df938059ab91c41d0785077b3329b793e051a2bf2eee64488b9f6029d9288c911052ec23ab3ab8c0561a2be1682dae

Author: fanquake

src/test/fuzz/fuzz.cpp

@@ -12,6 +12,7 @@
 
 const std::function<void(const std::string&)> G_TEST_LOG_FUN{};
 
+#if defined(__AFL_COMPILER)
 static bool read_stdin(std::vector<uint8_t>& data)
 {
     uint8_t buffer[1024];
@@ -23,6 +24,7 @@ static bool read_stdin(std::vector<uint8_t>& data)
     }
     return length == 0;
 }
+#endif
 
 // Default initialization: Override using a non-weak initialize().
 __attribute__((weak)) void initialize()
@@ -44,9 +46,9 @@ extern "C" int LLVMFuzzerInitialize(int* argc, char*** argv)
     return 0;
 }
 
-// Declare main(...) "weak" to allow for libFuzzer linking. libFuzzer provides
-// the main(...) function.
-__attribute__((weak)) int main(int argc, char** argv)
+// Generally, the fuzzer will provide main(), except for AFL
+#if defined(__AFL_COMPILER)
+int main(int argc, char** argv)
 {
     initialize();
 #ifdef __AFL_INIT
@@ -74,3 +76,4 @@ __attribute__((weak)) int main(int argc, char** argv)
 #endif
     return 0;
 }
+#endif