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

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8RJ1I


CVE: NA

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

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Felix Fu <fuzhen5@huawei.com>