LoongArch: kdump: Add memory reservation for old kernel

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I736HO



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After moving the old kernel memory reservation to elfcorehdr operation, avoid
the elfcorehdr space from being destroyed.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Change-Id: I94d5c38e24dd6d3ab3fddc3945580780d81e6922