Update fuzzing README for recent clang versions

Recent clang versions ship with libfuzzer, so there's no need to build
libfuzzer yourself. They also have a dedicated -fsanitize=fuzzer-no-link
flag and no longer support the sanitize flags described in the fuzzing
README. Update it to reflect all this.

Fixes #8768.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>

GH: #8891