RFR(XS): 8155735: use strings instead of Symbol* in JVMCI exception stubs

[Roland Schatz]

Hi,

Please review this small change:

webrev: http://cr.openjdk.java.net/~rschatz/JDK-8155735/webrev.00/
bug: https://bugs.openjdk.java.net/browse/JDK-8155735

This makes it possible to use the exception stubs without knowing about 
the hotspot-only concept of symbol pointers.

Thanks,
Roland