mm/dynamic_hugetlb: only compile PG_pool on X86_64 and ARM64

hulk inclusion
category: bugfix
bugzilla: 46904, https://gitee.com/openeuler/kernel/issues/I4YXOA



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When building with defconfig on arm32, we got a compile error:
./include/linux/page-flags-layout.h:95:2: error: #error "Not enough bits in page flags"
   95 | #error "Not enough bits in page flags"
      |  ^~~~~

Limit PG_pool to compile only on X86_64 and ARM64 to resolve this issue.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>