Makefile: update 'make fuzz-all' docs to reflect modern clang

Clang no longer produces a libFuzzer.a. Instead, you can include
libFuzzer by using -fsanitize=fuzzer. Therefore we should use that in
the example command for building fuzzers.

We also add -fsanitize=fuzzer-no-link to the CFLAGS to ensure that all
the required instrumentation is added when compiling git [1], and remove
 -fsanitize-coverage=trace-pc-guard as it is deprecated.

I happen to have tested with LLVM 11 - however -fsanitize=fuzzer appears
to work in a wide range of reasonably modern clangs.

(On my system: what used to be libFuzzer.a now lives under the following
 path, which is tricky albeit not impossible for a novice such as myself
 to find:
/usr/lib64/clang/11.0.0/lib/linux/libclang_rt.fuzzer-x86_64.a )

[1] https://releases.llvm.org/11.0.0/docs/LibFuzzer.html#fuzzer-usage

Signed-off-by: Andrzej Hunt <ajrhunt@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -3299,11 +3299,11 @@ cover_db_html: cover_db
 # are not necessarily appropriate for general builds, and that vary greatly
 # depending on the compiler version used.
 #
-# An example command to build against libFuzzer from LLVM 4.0.0:
+# An example command to build against libFuzzer from LLVM 11.0.0:
 #
 # make CC=clang CXX=clang++ \
-#      CFLAGS="-fsanitize-coverage=trace-pc-guard -fsanitize=address" \
-#      LIB_FUZZING_ENGINE=/usr/lib/llvm-4.0/lib/libFuzzer.a \
+#      CFLAGS="-fsanitize=fuzzer-no-link,address" \
+#      LIB_FUZZING_ENGINE="-fsanitize=fuzzer" \
 #      fuzz-all
 #
 FUZZ_CXXFLAGS ?= $(CFLAGS)