Fix fuzz target source path and binary name in gnutls.yaml (#750)

This is a tricky case because the old target path
(`/src/libtasn1/fuzz/libtasn1_gnutls_der_fuzzer.c`) is indeed a fuzz
target, but it was not used.

I found the new one and its corresponding binary with @jonathanmetzman's
trick:
1. Add a magic string (`const volatile char* build_id =
"BUILD_TRACKER_my_unique_identifier_123";`) to the fuzz target before
compilation.
2. Find the binary with the string (`find /out -type f -executable
-print0 | xargs -0 sh -c 'for file do if strings "$file" | grep
"BUILD_TRACKER_" >/dev/null; then basename "$file"; fi; done'`) after
compilation.

I reckon we can do this systematically and automatically:
1. Inject unique magic strings (e.g.,
`BUILD_TRACKER_my_unique_identifier_1`,
`BUILD_TRACKER_my_unique_identifier_2`) to all c/c++ files containing
`LLVMFuzzerTestOneInput`.
2. Match binaries in /out against fuzz target source files based on the
string.

benchmark-sets/all/gnutls.yaml

@@ -49,5 +49,5 @@
   "signature": "ssize_t gnutls_record_recv_seq(gnutls_session_t, void *, size_t, unsigned char *)"
 "language": "c++"
 "project": "gnutls"
-"target_name": "libtasn1_gnutls_der_fuzzer"
-"target_path": "/src/libtasn1/fuzz/libtasn1_gnutls_der_fuzzer.c"
+"target_name": "gnutls_private_key_parser_fuzzer"
+"target_path": "/src/gnutls/fuzz/gnutls_private_key_parser_fuzzer.c"