LoongArch: Define symbol 'fault' as a local label in fpu.S

LoongArch: Define symbol 'fault' as a local label in fpu.S

The initial aim is to silence the following objtool warnings:

  arch/loongarch/kernel/fpu.o: warning: objtool: _save_fp_context() falls through to next function fault()
  arch/loongarch/kernel/fpu.o: warning: objtool: _restore_fp_context() falls through to next function fault()
  arch/loongarch/kernel/fpu.o: warning: objtool: _save_lsx_context() falls through to next function fault()
  arch/loongarch/kernel/fpu.o: warning: objtool: _restore_lsx_context() falls through to next function fault()
  arch/loongarch/kernel/fpu.o: warning: objtool: _save_lasx_context() falls through to next function fault()
  arch/loongarch/kernel/fpu.o: warning: objtool: _restore_lasx_context() falls through to next function fault()

Currently, SYM_FUNC_START()/SYM_FUNC_END() defines the symbol 'fault' as
SYM_T_FUNC which is STT_FUNC, the objtool warnings are generated through
the following code:

tools/objtool/include/objtool/check.h:

static inline struct symbol *insn_func(struct instruction *insn)
{
	struct symbol *sym = insn->sym;

	if (sym && sym->type != STT_FUNC)
		sym = NULL;

	return sym;
}

tools/objtool/check.c:

static int validate_branch(struct objtool_file *file, struct symbol *func,
			   struct instruction *insn, struct insn_state state)
{
	...
		if (func && insn_func(insn) && func != insn_func(insn)->pfunc) {
	...
			WARN("%s() falls through to next function %s()",
			     func->name, insn_func(insn)->name);
			return 1;
		}
	...
}

We can see that the fixup can be a local label in the following code:

arch/loongarch/include/asm/asm-extable.h:
	.pushsection	__ex_table, "a";		\
	.balign		4;				\
	.long		((insn) - .);			\
	.long		((fixup) - .);			\
	.short		(type);				\
	.short		(data);				\
	.popsection;

	.macro		_asm_extable, insn, fixup
	__ASM_EXTABLE_RAW(\insn, \fixup, EX_TYPE_FIXUP, 0)
	.endm

Like arch/loongarch/lib/*.S, just define the symbol 'fault' as a local
label in fpu.S.

Before:

$ readelf -s arch/loongarch/kernel/fpu.o | awk -F: /fault/'{print $2}'
 000000000000053c     8 FUNC    GLOBAL DEFAULT    1 fault

After:

$ readelf -s arch/loongarch/kernel/fpu.o | awk -F: /fault/'{print $2}'
 000000000000053c     0 NOTYPE  LOCAL  DEFAULT    1 .L_fpu_fault

Co-developed-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>