Merge #19366: tests: Provide main(...) function in fuzzer. Allow building uninstrumented harnesses with --enable-fuzz.

1087807 tests: Provide main(...) function in fuzzer (practicalswift)

Pull request description:

  Provide `main(...)` function in fuzzer. Allow building uninstrumented harnesses with only `--enable-fuzz`.

  This PR restores the behaviour to how things worked prior to #18008. #18008 worked around an macOS specific issue but did it in a way which unnecessarily affected platforms not in need of the workaround :)

  Before this patch:

  ```
  # Build uninstrumented fuzzing harness (no libFuzzer/AFL/other-fuzzer-instrumentation)
  $ ./configure --enable-fuzz
  $ make
    CXXLD    test/fuzz/span
  /usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
  (.text+0x20): undefined reference to `main'
  collect2: error: ld returned 1 exit status
  Makefile:7244: recipe for target 'test/fuzz/span' failed
  make[2]: *** [test/fuzz/span] Error 1
  make[2]: *** Waiting for unfinished jobs....
  $
  ```

  After this patch:

  ```
  # Build uninstrumented fuzzing harness (no libFuzzer/AFL/other-fuzzer-instrumentation)
  $ ./configure --enable-fuzz
  $ make
  $ echo foo | src/test/fuzz/span
  $
  ```

  The examples above show the change in non-macOS functionality. macOS functionality is unaffected by this patch.

ACKs for top commit:
  MarcoFalke:
    ACK 1087807

Tree-SHA512: 9c16ea32ffd378057c4fae9d9124636d11e3769374d340f68a1b761b9e3e3b8a33579e60425293c96b8911405d8b96ac3ed378e669ea4c47836af06892aca73d

Author: MarcoFalke

src/test/fuzz/fuzz.cpp

@@ -12,7 +12,16 @@
 
 const std::function<void(const std::string&)> G_TEST_LOG_FUN{};
 
-#if defined(__AFL_COMPILER)
+// Decide if main(...) should be provided:
+// * AFL needs main(...) regardless of platform.
+// * macOS handles __attribute__((weak)) main(...) poorly when linking
+//   against libFuzzer. See https://github.com/bitcoin/bitcoin/pull/18008
+//   for details.
+#if defined(__AFL_COMPILER) || !defined(MAC_OSX)
+#define PROVIDE_MAIN_FUNCTION
+#endif
+
+#if defined(PROVIDE_MAIN_FUNCTION)
 static bool read_stdin(std::vector<uint8_t>& data)
 {
     uint8_t buffer[1024];
@@ -44,9 +53,8 @@ extern "C" int LLVMFuzzerInitialize(int* argc, char*** argv)
     return 0;
 }
 
-// Generally, the fuzzer will provide main(), except for AFL
-#if defined(__AFL_COMPILER)
-int main(int argc, char** argv)
+#if defined(PROVIDE_MAIN_FUNCTION)
+__attribute__((weak)) int main(int argc, char** argv)
 {
     initialize();
 #ifdef __AFL_INIT