ZGC: Explicitly use 2M large pages

ZGC requires large pages to be 2M. However, ZGC doesn't explicitly asks for this page size and instead relies on the default large pages size for the system to be 2M. On systems where this is not true, ZGC will fails with an error message. To avoid this, ZGC should explicitly ask for 2M large pages and not rely on the system default. Furthermore, ZGC currently ignores -XX:LargePageSizeInBytes. ZGC should fails with an error message if it's specified to something other than 2M.