objtool/loongarch: Add unwind hints in prepare_frametrace()

objtool/loongarch: Add unwind hints in prepare_frametrace()

If 'regs' points to a local stack variable, prepare_frametrace() stores
all registers to the stack.  This confuses objtool as it expects them to
be restored from the stack later.

The stores don't affect stack tracing, so use unwind hints to hide them
from objtool.

Fixes the following warnings:

  arch/loongarch/kernel/traps.o: warning: objtool: show_stack+0xe0: stack state mismatch: reg1[22]=-1+0 reg2[22]=-2-160
  arch/loongarch/kernel/traps.o: warning: objtool: show_stack+0xe0: stack state mismatch: reg1[23]=-1+0 reg2[23]=-2-152

Fixes: cb8a2ef0848c ("LoongArch: Add ORC stack unwinder support")
Reported-by: kernel test robot <lkp@intel.com>
Tested-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/270cadd8040dda74db2307f23497bb68e65db98d.1743481539.git.jpoimboe@kernel.org
Closes: https://lore.kernel.org/oe-kbuild-all/202503280703.OARM8SrY-lkp@intel.com/