Refactor FileStackNode::printBacktrace from recursive to iterative

[Rangi42]

This avoids a potential stack overflow for very long backtraces, or for corrupt object files with cyclic backtraces

[Rangi42]

I'm fuzzing RGBLINK with this refactoring, as well as a guard against all crashes due to OOM (since we want to investigate crashes due to memory unsafety or other logic bugs, usually from corrupt object files that RGBASM would never output):

-int main(int argc, char *argv[]) {
+int tryMain(int argc, char *argv[]) {
     ...
 }
+
+int main(int argc, char *argv[]) {
+    try {
+        return tryMain(argc, argv);
+    } catch (std::bad_alloc const &) {
+        fatal("Out of memory");
+    }
+}

(Regarding use of exceptions if this makes it into production, see https://mropert.github.io/2025/12/03/enum_class_error_codes_part3/ -- and bad_alloc is the only exception we'd be handling anyway.)