RFR: 8218748: AARCH64: String::compareTo intrinsic documentation and maintenance improvement

[Dmitrij Pochepko]

Hi all.

Please review an enhancement change "8218748: AARCH64: String::compareTo 
intrinsic documentation and maintenance improvement".

RFE: https://bugs.openjdk.java.net/browse/JDK-8218748
webrev: http://cr.openjdk.java.net/~dpochepk/8218748/webrev/

Changes:
- Added documentation for compareTo implementation.
- Removed duplicated code and using existing code instead.
-- compare_string_16_bytes_same is used for tail handling in 
generate_compare_long_string_same_encoding .
-- DIFF block code is reused for DIFF2 block in 
generate_compare_long_string_same_encoding.
- Registers and labels are partially renamed to better reflect code 
actions.

This patch is dependent on JDK-8218966 and should be applied on top of it.

Testing:
- jck
- hotspot jtreg compiler/*, runtime/* and gc/* tests. Including 
AARCH64-implementation-specific compareTo tests introduced in 
JDK-8218966 fix
- jdk jtreg tier1-3 tests

No regressions were found.

I'd like to thank Pengfei Li (Pengfei.Li at arm.com) for pre-review and 
additional testing.

Thanks,
Dmitrij