Update the fuzz README to provide info about the stdc++ requirements

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22368)

fuzz/README.md

@@ -32,6 +32,34 @@ to the `libFuzzer` library file while configuring; this is represented as
             enable-ssl3 enable-ssl3-method enable-nextprotoneg \
             --debug
 
+Clang uses the gcc libstdc++ library so this must also be installed. You can
+check which version of gcc clang is using like this:
+
+    $ clang --verbose
+    Ubuntu clang version 14.0.0-1ubuntu1.1
+    Target: x86_64-pc-linux-gnu
+    Thread model: posix
+    InstalledDir: /usr/bin
+    Found candidate GCC installation: /usr/bin/../lib/gcc/i686-linux-gnu/12
+    Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
+    Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/11
+    Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/12
+    Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/12
+    Candidate multilib: .;@m64
+    Selected multilib: .;@m64
+
+So, in the above example clang is using gcc version 12. Ensure that the selected
+gcc version has the relevant libstdc++ files installed:
+
+    $ ls /usr/lib/gcc/x86_64-linux-gnu/12 | grep stdc++
+    libstdc++.a
+    libstdc++fs.a
+    libstdc++.so
+
+On Ubuntu for gcc-12 this requires the libstdc++-12-dev package installed.
+
+    $ sudo apt-get install libstdc++-12-dev
+
 Compile:
 
     sudo apt-get install make