RFR (XS) 8184762: ZapStackSegments should use optimized memset

[Aleksey Shipilev]

https://bugs.openjdk.java.net/browse/JDK-8184762

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. The choice of 0xCA is arbitrary, and open for discussion.

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

Testing: hotspot_tier1 (Linux x86_64 / fastdebug)

Thanks,
-Aleksey