LoongArch: Fix the _stext symbol address

_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>
diff --git a/arch/loongarch/kernel/vmlinux.lds.S b/arch/loongarch/kernel/vmlinux.lds.S
index 78311a6..69c76f2 100644
--- a/arch/loongarch/kernel/vmlinux.lds.S
+++ b/arch/loongarch/kernel/vmlinux.lds.S
@@ -37,6 +37,7 @@
 	HEAD_TEXT_SECTION
 
 	. = ALIGN(PECOFF_SEGMENT_ALIGN);
+	_stext = .;
 	.text : {
 		TEXT_TEXT
 		SCHED_TEXT