Shenandoah critical native tests should only be ran on x86_64 platforms

[Zhengyu Gu]

Shenandoah critical native support is only implemented for x86_64, so 
only corresponding tests should only be ran on those platform.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/critical-native-tests/webrev.00/

Test:
   Two critical native tests on Linux x64


Thanks,

-Zhengyu