LoongArch: Fix some build warnings with W=1

LoongArch: Fix some build warnings with W=1

There are some building warnings when building LoongArch kernel with W=1
as following, this patch fixes them.

arch/loongarch/kernel/acpi.c:284:13: warning: no previous prototype for ‘acpi_numa_arch_fixup’ [-Wmissing-prototypes]
  284 | void __init acpi_numa_arch_fixup(void) {}
      |             ^~~~~~~~~~~~~~~~~~~~
arch/loongarch/kernel/time.c:32:13: warning: no previous prototype for ‘constant_timer_interrupt’ [-Wmissing-prototypes]
   32 | irqreturn_t constant_timer_interrupt(int irq, void *data)
      |             ^~~~~~~~~~~~~~~~~~~~~~~~
arch/loongarch/kernel/traps.c:496:25: warning: no previous prototype for 'do_fpe' [-Wmissing-prototypes]
  496 | asmlinkage void noinstr do_fpe(struct pt_regs *regs
      |                         ^~~~~~
arch/loongarch/kernel/traps.c:813:22: warning: variable ‘opcode’ set but not used [-Wunused-but-set-variable]
  813 |         unsigned int opcode;
      |                      ^~~~~~
arch/loongarch/kernel/signal.c:895:14: warning: no previous prototype for ‘get_sigframe’ [-Wmissing-prototypes]
  895 | void __user *get_sigframe(struct ksignal *ksig, struct pt_regs *regs,
      |              ^~~~~~~~~~~~
arch/loongarch/kernel/syscall.c:21:40: warning: initialized field overwritten [-Woverride-init]
   21 | #define __SYSCALL(nr, call)     [nr] = (call),
      |                                        ^
arch/loongarch/kernel/syscall.c:40:14: warning: no previous prototype for ‘do_syscall’ [-Wmissing-prototypes]
   40 | void noinstr do_syscall(struct pt_regs *regs)
      |              ^~~~~~~~~~
arch/loongarch/kernel/smp.c:502:17: warning: no previous prototype for ‘start_secondary’ [-Wmissing-prototypes]
  502 | asmlinkage void start_secondary(void)
      |                 ^~~~~~~~~~~~~~~
arch/loongarch/kernel/process.c:309:15: warning: no previous prototype for ‘arch_align_stack’ [-Wmissing-prototypes]
  309 | unsigned long arch_align_stack(unsigned long sp)
      |               ^~~~~~~~~~~~~~~~
arch/loongarch/kernel/topology.c:13:5: warning: no previous prototype for ‘arch_register_cpu’ [-Wmissing-prototypes]
   13 | int arch_register_cpu(int cpu)
      |     ^~~~~~~~~~~~~~~~~
arch/loongarch/kernel/topology.c:27:6: warning: no previous prototype for ‘arch_unregister_cpu’ [-Wmissing-prototypes]
   27 | void arch_unregister_cpu(int cpu)
      |      ^~~~~~~~~~~~~~~~~~~
arch/loongarch/kernel/module-sections.c:103:5: warning: no previous prototype for ‘module_frob_arch_sections’ [-Wmissing-prototypes]
  103 | int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~
arch/loongarch/mm/hugetlbpage.c:56:5: warning: no previous prototype for ‘is_aligned_hugepage_range’ [-Wmissing-prototypes]
   56 | int is_aligned_hugepage_range(unsigned long addr, unsigned long len)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>