Don't use memset to initialize arrays of MemoryUsage in memoryManager.cpp

gcc8 warnings:

In multiple places there are errors reported like:

src/hotspot/share/services/memoryManager.cpp:172:40: error: ‘void* memset(void*, int, size_t)’ clearing an object of non-trivial type ‘class MemoryUsage’; use assignment or value-initialization instead [-Werror=class-memaccess]
   memset(_before_gc_usage_array, 0, len);
                                        ^

These could be fixed using placement new, e.g.

  ::new (_before_gc_usage_array) MemoryUsage()