RFR(XS): JDK-8214272: Don't use memset to initialize arrays of MemoryUsage in memoryManager.cpp

[Dmitry Chuyko]

Hello,

GCC 8 treats zeroing of non-trivial objects array with memset() as an 
error. Please review a small patch that puts 'placement new' instead of 
memset() in GCStatInfo::clear().
Re-sending to hotspot-gc-dev.


rfe: https://bugs.openjdk.java.net/browse/JDK-8214272
webrev: http://cr.openjdk.java.net/~dchuyko/8214272/webrev.00/
testing: dev-submit

-Dmitry