RFR: 8218749 - AARCH64: String compress intrinsic documentation and maintenance improvement

[Dmitrij Pochepko]

Hi all,

please review patch for JDK-8218749: AARCH64: String compress intrinsic 
documentation and maintenance improvement
webrev: http://cr.openjdk.java.net/~dpochepk/8218749/webrev.01/

This patch adds documentation with test and moves equivalent code 
generation into separate method. Documentation is both added into 
documentation block and inlined into code.

Testing:

- hotspot jtreg tests: compiler/*, gc/* and runtime/*
- jdk jtreg tests: tier1-tier3
- jck tests

No regression found.

I'd like to thank Pengfei Li for help in pre-review.

CR: https://bugs.openjdk.java.net/browse/JDK-8218749

Thanks,
Dmitrij