LoongArch: Use SYM_CODE_* to annotate exception handlers

As described in include/linux/linkage.h,

  FUNC -- C-like functions (proper stack frame etc.)
  CODE -- non-C code (e.g. irq handlers with different, special stack etc.)

  SYM_FUNC_{START, END} -- use for global functions
  SYM_CODE_{START, END} -- use for non-C (special) functions

So use SYM_CODE_* to annotate exception handlers.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>