cmake: remove need to add "-fsanitize=fuzzer" flag for fuzzers Right now, users are being instrucded to add the "-DCMAKE_EXE_LINKER_FLAGS=-fsanitize=fuzzer" flag when they want to build our fuzzers. This is error-prone and user unfriendly. Instead, just add the flag to our fuzzers' build instructions so that it happens automatically. Adjust the README accordingly.