doc: update fuzz instructions when on macOS

Default linker on macOS does not work with recent versions of LLVM. Updated the instructions for fuzzing to use lld instead.

doc/fuzzing.md

@@ -153,13 +153,18 @@ You may also need to take care of giving the correct path for `clang` and
 `clang++`, like `CC=/path/to/clang CXX=/path/to/clang++` if the non-systems
 `clang` does not come first in your path.
 
+Currently the default linker on macOS `ld` does not work with recent versions of `llvm` so you will need to install llvm's linker `lld` and use it with `-fuse-ld=lld`.
+
 Full configuration step that was tested on macOS with `brew` installed `llvm`:
 
 ```sh
+$ brew install llvm
+$ brew install lld
 $ cmake --preset=libfuzzer \
    -DCMAKE_C_COMPILER="$(brew --prefix llvm)/bin/clang" \
    -DCMAKE_CXX_COMPILER="$(brew --prefix llvm)/bin/clang++" \
-   -DAPPEND_LDFLAGS=-Wl,-no_warn_duplicate_libraries
+   -DAPPEND_LDFLAGS=-Wl,-no_warn_duplicate_libraries \
+   -DCMAKE_CXX_FLAGS="-fuse-ld=lld"
 ```
 
 Read the [libFuzzer documentation](https://llvm.org/docs/LibFuzzer.html) for more information. This [libFuzzer tutorial](https://github.com/google/fuzzing/blob/master/tutorial/libFuzzerTutorial.md) might also be of interest.