[8u] RFR (XS) 8269859: BacktraceBuilder._cprefs needs to be accessed as unsigned short

[Aleksey Shipilev]

Bug:
   https://bugs.openjdk.java.net/browse/JDK-8269859

This is a 8u-specific bug that I found with source code inspection while working on a related 
backport. With unmodified JDK, it is hard to reproduce, because it involves large class that 
undergoes redefinition. Yet, with a simple VM change, you can reproduce the assert 100% reliably.

See the bug report for more details.

8u webrev:
   https://cr.openjdk.java.net/~shade/8269859/webrev.8u.01/

Testing: test from 8194246 with/without the fix, with/without the cpref-exposing patch; tier1 
with/without the cpref-exposing patch.

-- 
Thanks,
-Aleksey