In the fastdebug and debug builds, libhpi.so may call our HPI panic() callback during a sysAssert() failure.  Currently the callback is unimplemented and results in a call to Unimplemented(), masking the true cause of the failure.  We should provide an implementation for the sake of better error reporting.

For further background see bug 6502237.
I don't think hpi should call back to the VM to die anyway, but call regular unix assert, but we should give an error and add that the bug is in hpi not the VM.

This is annoying that so many linux tests intermittently fail with this hpi bug.
I changed this hpi_panic from Unimplemented() crash to a warning(Unimplemented) because bug number 6626677 was affecting testing.  This is safe for this particular bug anyway.

As part of this fix, the crash should be restored in the VM for jdk7 only (there's apparently a gate we have in the vm) after bug 6626677 is fixed.  If the jdk fix is backported to jdk 6 we can enable the crash unconditionally in the VM, but after HS13.