RFR(T) 8218566: NMT: missing memory tag for assert poison page

[zgu at redhat.com]

Please review the trivial change to tag assert poison page.

Bug: https://bugs.openjdk.java.net/browse/JDK-8218566
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8218566/webrev.00/

Test:
  hotspot_nmt on Linux x64 (fastdebug and release),
  passed all except runtime/NMT/CheckForProperDetailStackTrace.java  
  (JDK-8218458) with release build.

Thanks,

-Zhengyu