ZapStackSegments should use optimized memset

fastdebug builds performance is important to minimize testing time. ZapStackSegments is trueInDebug, and used to zap vm/utilities/stack.* segments as they get recycled. However, that zapping uses the handrolled memset, which shows up in profiles with individual stores. It seems better to use Copy::fill_to_bytes for this, which will call into platform-specific memset. Additionally, this would be a good time to introduce its zapped pattern to globalDefinitions.

Webrev:
 http://cr.openjdk.java.net/~shade/8184762/webrev.01/