After merging +num64, the 64-bit sanitizer builds show that Vim doesn't
buffer the user from C's UB in signed arithmetic. Upstream doesn't
appear to be [interested] in fixing the issue, so suppress UBSAN until
someone decides to fix the problem.
N.B., the problem existed before but went unnoticed since the sanitizer
builds weren't being run in 32-bit mode.
[interested]: https://groups.google.com/d/topic/vim_dev/_tqf8eQy5eA/discussion
