RFR: 8062247: Allow WhiteBox test to access JVM offsets

[Yumin Qi]

Please review

bug: https://bugs.openjdk.java.net/browse/JDK-8062247
webrev: http://cr.openjdk.java.net/~minqi/8062247/webrev00/

Summary: Internal test failed since the variable offsets changed in 
hotspot. The way to get offset in the test is hard-coded. To reduce the 
risk of future changes of hotspot offsets, the fix add a WhiteBox API 
function to get a map for FileMapHeaderInfo, which return the members' 
offsets in a Hashtable.

Tests: JPRT, jtreg.

Thanks
Yumin