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>

arch/loongarch/kernel/head.S

@@ -14,8 +14,6 @@
 
 	__REF
 
-SYM_ENTRY(_stext, SYM_L_GLOBAL, SYM_A_NONE)
-
 SYM_CODE_START(kernel_entry)			# kernel entry point
 
 	/* Config direct window and set PG */

arch/loongarch/kernel/vmlinux.lds.S

@@ -37,6 +37,7 @@ SECTIONS
 	HEAD_TEXT_SECTION
 
 	. = ALIGN(PECOFF_SEGMENT_ALIGN);
+	_stext = .;
 	.text : {
 		TEXT_TEXT
 		SCHED_TEXT