5042 stop using deprecated atomic functions

*** 372,382 ****
  void
  fm_panic(const char *format, ...)
  {
          va_list ap;
  
!         (void) casptr((void *)&fm_panicstr, NULL, (void *)format);
  #if defined(__i386) || defined(__amd64)
          fastreboot_disable_highpil();
  #endif /* __i386 || __amd64 */
          va_start(ap, format);
          vpanic(format, ap);
--- 372,382 ----
  void
  fm_panic(const char *format, ...)
  {
          va_list ap;
  
!         (void) atomic_cas_ptr((void *)&fm_panicstr, NULL, (void *)format);
  #if defined(__i386) || defined(__amd64)
          fastreboot_disable_highpil();
  #endif /* __i386 || __amd64 */
          va_start(ap, format);
          vpanic(format, ap);