LoongArch: fix vmlinux ld error

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6BWFP



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When CONFIG_DEBUG_INFO_BTF is enabled, ld load error. To
fix this issue, we should use -mdirect-extern-access.

Signed-off-by: Xing Li <lixing@loongson.cn>
Signed-off-by: Hongchen Zhang <zhanghongchen@loongson.cn>
Change-Id: I698169b2af48369d531ef3aa4f9c53b97096da83