JDK-8350441 introduced Mapped Cache for ZGC. However, the constructor of `ZMappedCache` uses brace-initialization for `_size_class_lists`, i.e. a `ZList` array. `ZList` is a class with a deleted copy constructor and an explicit destructor, and when its array is brace-initialized in the constructor, it triggers GCC bug 63707 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63707). The bug causes compilation error of ZGC on GCC versions 10.1 to 10.2. Considering OpenJDK compilation is still requires GCC 10 or higher, this should be recorded as a bug.

This patch uses value-initialization for `_size_class_lists` instead of brace-initialization, which should be semantically equivalent and work on all GCC versions.