LoongArch: Rework the arch_kgdb_breakpoint() implementation

[ Upstream commit 29c92a41c6d2879c1f62220fe4758dce191bb38f ]

The arch_kgdb_breakpoint() function defines the kgdb_breakinst symbol
using inline assembly.

1. There's a potential issue where the compiler might inline
arch_kgdb_breakpoint(), which would then define the kgdb_breakinst
symbol multiple times, leading to a linker error.

To prevent this, declare arch_kgdb_breakpoint() as noinline.

Fix follow error with LLVM-19 *only* when LTO_CLANG_FULL:
    LD      vmlinux.o
  ld.lld-19: error: ld-temp.o <inline asm>:3:1: symbol 'kgdb_breakinst' is already defined
  kgdb_breakinst: break 2
  ^

2. Remove "nop" in the inline assembly because it's meaningless for
LoongArch here.

3. Add "STACK_FRAME_NON_STANDARD" for arch_kgdb_breakpoint() to avoid
the objtool warning.

Fixes: e14dd07696 ("LoongArch: Add basic KGDB & KDB support")
Tested-by: Binbin Zhou <zhoubinbin@loongson.cn>
Co-developed-by: Winston Wen <wentao@uniontech.com>
Signed-off-by: Winston Wen <wentao@uniontech.com>
Co-developed-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: Yuli Wang <wangyuli@uniontech.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/loongarch/kernel/kgdb.c

@@ -8,6 +8,7 @@
 #include <linux/hw_breakpoint.h>
 #include <linux/kdebug.h>
 #include <linux/kgdb.h>
+#include <linux/objtool.h>
 #include <linux/processor.h>
 #include <linux/ptrace.h>
 #include <linux/sched.h>
@@ -224,13 +225,13 @@ void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long pc)
 	regs->csr_era = pc;
 }
 
-void arch_kgdb_breakpoint(void)
+noinline void arch_kgdb_breakpoint(void)
 {
 	__asm__ __volatile__ (			\
 		".globl kgdb_breakinst\n\t"	\
-		"nop\n"				\
 		"kgdb_breakinst:\tbreak 2\n\t"); /* BRK_KDB = 2 */
 }
+STACK_FRAME_NON_STANDARD(arch_kgdb_breakpoint);
 
 /*
  * Calls linux_debug_hook before the kernel dies. If KGDB is enabled,