RFR: 8358104: Fix ZGC compilation error on GCC 10.2

[Qizheng Xing]

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). A short example to reproduce this bug: https://godbolt.org/z/3397bxc73

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.

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Commit messages:
 - Fix ZGC compilation error on GCC 10.2

Changes: https://git.openjdk.org/jdk/pull/25536/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25536&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358104
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25536.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25536/head:pull/25536

PR: https://git.openjdk.org/jdk/pull/25536