Suppress thread-unsafe library function calls in single-threaded mode

[sim642]

We don't record accesses in single-threaded mode for race detection at all (they don't even show with allglobs) to avoid overhead on single-threaded programs. However, thread-unsafe library function calls do get recorded in single-threaded mode, but there's no need to do that either.

#include <stdlib.h>

int main() {
  rand();
  return 0;
}

$ ./goblint bar.c --enable allglobs 
[Info][Deadcode] Logical lines of code (LLoC) summary:
  live: 3
  dead: 0
  total lines: 3
[Success][Race] Memory location rand (safe): (/usr/include/stdlib.h:454:12-454:24)
  call with [mhp:{tid=[main]}, multi:false, thread:[main]] (conf. 110)  (exp: rand) (bar.c:4:3-4:9)
[Info][Race] Memory locations race summary:
  safe: 1
  vulnerable: 0
  unsafe: 0
  total memory locations: 1

There should be no race summary printed for this program at all.