efi/libstub: arm64: support strchr function for EFI stub

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5J0X7


CVE: NA

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Since arm64 does not use a builtin decompressor, the EFI stub is built
into the kernel proper. So instead, separate the contents of libstub
and its dependencies, by putting them into their own namespace by
prefixing all of its symbols with __efistub.This way, we have tight
control over what parts of the kernel proper are referenced by the stub.

Follow the rules and support strchr function for EFI stub, it will be
used to parse cmdline args.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Wang Weiyang <wangweiyang2@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>