LoongArch: kexec: Add compatibility with old interfaces

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



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Old interface:
a0 = argc, a1 = argv, a2 = envp

New interface:
a0 = efi flag, a2 = cmdline, a3 = systemtab

The following interfaces are not supported:
a0 = efi flag, a2 = small fdt

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Change-Id: If490ebe181ea75c955b93a69d28994f326938d93