fix(x86_64): fix TraceSubscriber emitting enter instead of exit (#296)

Currently, the `x86_64` `TraceSubscriber` incorrectly writes a duplicate
`Enter` message to the serial port when a span is exited, instead of an
`Exit` message. This commit fixes that.

Fixes #290

platforms/x86_64/core/src/trace.rs

@@ -87,7 +87,7 @@ where
     }
 
     fn exit(&self, span: &span::Id) {
-        with_serial(|serial| serial.enter(span));
+        with_serial(|serial| serial.exit(span));
     }
 
     fn record_follows_from(&self, _: &span::Id, _: &span::Id) {