*: 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;

src/nvim/eval/executor.c

@@ -25,7 +25,7 @@ char *e_listidx = N_("E684: list index out of range: %" PRId64);
 /// @return OK or FAIL.
 int eexe_mod_op(typval_T *const tv1, const typval_T *const tv2,
                 const char *const op)
-  FUNC_ATTR_NONNULL_ALL
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_NO_SANITIZE_UNDEFINED
 {
   // Can't do anything with a Funcref, a Dict or special value on the right.
   if (tv2->v_type != VAR_FUNC && tv2->v_type != VAR_DICT) {