tests: Don't assume presence of __builtin_mul_overflow in MultiplicationOverflow(...) fuzzing harness

Summary:
Backport of core PR18393.

Depends on D8967.

Test Plan:
I cannot reproduce the issue myself, but I can check it doesn't break
the fuzzer:

ninja bitcoin-fuzzers
./test/fuzz/test_runner.py <path_to_corpus> multiplication_overflow

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8969