RFR 8150778: Reduce Throwable.getStackTrace() calls to the JVM

[Coleen Phillimore]

Summary: replace JVM_GetStackTraceDepth and JVM_GetStackTraceElement, 
with JVM_GetStackTraceElements that gets all the elements in the 
StackTraceElement[]

These improvements were found during the investigation for replacing 
Throwable with the StackWalkAPI.   This change also adds iterator for 
BacktraceBuilder to make changing format of backtrace easier.

Tested with -testset core, RBT nightly hotspot nightly tests on all 
platforms, and jck tests on linux x64.  Compatibility request is approved.

open webrev at http://cr.openjdk.java.net/~coleenp/8150778_jdk/
open webrev at http://cr.openjdk.java.net/~coleenp/8150778_hotspot
bug link https://bugs.openjdk.java.net/browse/JDK-8150778

Thanks,
Coleen