fixup "Don't free regex objects in exit-time finalizer calls (#57834)"

Author: IanButterworth

base/regex.jl

@@ -40,8 +40,9 @@ mutable struct Regex <: AbstractPattern
         re = compile(new(pattern, compile_options, match_options, C_NULL))
         finalizer(re) do re
             # don't free during exit because tasks may still be running and
-            # using it. Issue #57817
-            during_exit = Base._atexit_hooks_finished
+            # using it. Issue #57817. During sysimage creation _atexit_hooks_finished
+            # is not defined but threads aren't running so just always run
+            during_exit = @isdefined(_atexit_hooks_finished) && _atexit_hooks_finished
             if re.regex != C_NULL && !during_exit
                 PCRE.free_re(re.regex)
             end