RFR(XS) : 8144695 : --disable-warnings-as-errors does not work for HotSpot build

[Igor Ignatyev]

> http://cr.openjdk.java.net/~iignatyev/8144695/webrev.00/
11 lines changed: 4 ins; 0 del; 7 mod; 

Hi,

could you please review the fix for 8144695[1]?

8141543[1] propagated --disable-warnings-as-errors to hotspot build by setting WARNINGS_ARE_ERRORS to empty line. however in hotspot makefiles there are places where this variable is set explicitly.
the fix changes = operator to ?= operator and updates adlc.make to use WARNINGS_ARE_ERRORS if it’s defined.

[1] https://bugs.openjdk.java.net/browse/JDK-8144695
[2] https://bugs.openjdk.java.net/browse/JDK-8141543

Thanks,
— Igor