RFR(XXS) 8199741: NMT: Tag safepoint polling pages

[Zhengyu Gu]

A trivial fix to add NMT tag to Safepoint poll pages.

Bug: https://bugs.openjdk.java.net/browse/JDK-8199741
Webrev: http://cr.openjdk.java.net/~zgu/8199741/webrev.00/

Test:
   Eyeball NMT output on Linux x64.

Thanks,

-Zhengyu