Fix rb_eSystemExit raised in Ractor

[Bug #21505]

Previously `Ractor.new { exit }.join` would hang because SystemExit was
special cased.

This commit updates this to take the same path as other exceptions,
which wraps the exception in a Ractor::RemoteError and does not end up
exiting the main Ractor. I don't know if that's what this should do, but
I think it's a reasonable behaviour as calling exit() in a Ractor is
odd.

    in 'Ractor#join': thrown by remote Ractor. (Ractor::RemoteError)
       from -e:1:in '<main>'
    in 'Kernel#exit': exit (SystemExit)
            from -e:1:in 'block in <main>'

bootstraptest/test_ractor.rb

@@ -358,6 +358,32 @@ assert_equal 'ok', %q{
   r.value
 }
 
+# SystemExit from a Ractor is re-raised
+# [Bug #21505]
+assert_equal '[SystemExit, "exit", true]', %q{
+  r = Ractor.new { exit }
+  begin
+    r.value
+  rescue Ractor::RemoteError => e
+    [e.cause.class,   #=> RuntimeError
+     e.cause.message, #=> 'ok'
+     e.ractor == r]   #=> true
+  end
+}
+
+# SystemExit from a Thread inside a Ractor is re-raised
+# [Bug #21505]
+assert_equal '[SystemExit, "exit", true]', %q{
+  r = Ractor.new { Thread.new { exit }.join }
+  begin
+    r.value
+  rescue Ractor::RemoteError => e
+    [e.cause.class,   #=> RuntimeError
+     e.cause.message, #=> 'ok'
+     e.ractor == r]   #=> true
+  end
+}
+
 # threads in a ractor will killed
 assert_equal '{ok: 3}', %q{
   Ractor.new Ractor.current do |main|

thread.c

@@ -703,6 +703,10 @@ thread_start_func_2(rb_thread_t *th, VALUE *stack_start)
             /* fatal error within this thread, need to stop whole script */
         }
         else if (rb_obj_is_kind_of(errinfo, rb_eSystemExit)) {
+            if (th->invoke_type == thread_invoke_type_ractor_proc) {
+                rb_ractor_atexit_exception(th->ec);
+            }
+
             /* exit on main_thread. */
         }
         else {