LoongArch: Fix the _stext symbol address

mainline inclusion
from mainline-v5.19-rc4
commit 92264f2d
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OHOB


CVE: NA

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_stext means the start of .text section (see __is_kernel_text()), but we
put its definition in .ref.text by mistake. Fix it by defining it in the
vmlinux.lds.S.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>