*: Disable UBSAN for VimL arithmetic implementation

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

src/nvim/eval.c

@@ -3934,7 +3934,7 @@ eval6 (
     typval_T *rettv,
     int evaluate,
     int want_string              /* after "." operator */
-)
+) FUNC_ATTR_NO_SANITIZE_UNDEFINED
 {
   typval_T var2;
   int op;