system/excpt: let the OS handle termination on signal (#16712)

changelog.md

@@ -159,6 +159,14 @@ provided by the operating system.
   `ValueError` when the real command line is not available. `parseopt` was
   previously excluded from `prelude` for JS, as it could not be imported.
 
+- On POSIX systems, the default signal handlers used for Nim programs (it's
+  used for printing the stacktrace on fatal signals) will now re-raise the
+  signal for the OS default handlers to handle.
+
+  This lets the OS perform its default actions, which might include core
+  dumping (on select signals) and notifying the parent process about the cause
+  of termination.
+
 ## Language changes
 
 - `nimscript` now handles `except Exception as e`.