Make Xerces DatatypeException stackless

Profiling of SPECjvm2008:xml.validation shows there are lots of InvalidateDataTypeValueExceptions are thrown from Xerces. See screenshot for async-profiler stack trace.

It does not look like those exceptions leak outside javax.xml. The uses of exception or its subclasses (`InvalidDatatypeValueException`, `InvalidDatatypeFacetException`) take the form of:

```
                catch (InvalidDatatypeValueException ex) {
                    fErrorReporter.reportError(XMLMessageFormatter.XML_DOMAIN,
                                               ex.getKey(),
                                               ex.getArgs(),
                                               XMLErrorReporter.SEVERITY_ERROR );
                }
```

...so they don't seem to use the stack traces: only key, args, message is used. Which means we can turn `DatatypeException` stackless, and save some cycles on throws.

This improves xml.validation performance for about +3%.