libgcc: add forward declarations for generic library routines

With W=1 on platforms that use the generic gcc library routines
(csky/loongarch/mips/riscv/sh/xtensa):

    lib/ashldi3.c:9:19: warning: no previous prototype for '__ashldi3' [-Wmissing-prototypes]
	9 | long long notrace __ashldi3(long long u, word_type b)
	  |                   ^~~~~~~~~
      CC      lib/ashrdi3.o
    lib/ashrdi3.c:9:19: warning: no previous prototype for '__ashrdi3' [-Wmissing-prototypes]
	9 | long long notrace __ashrdi3(long long u, word_type b)
	  |                   ^~~~~~~~~
      CC      lib/cmpdi2.o
    lib/cmpdi2.c:9:19: warning: no previous prototype for '__cmpdi2' [-Wmissing-prototypes]
	9 | word_type notrace __cmpdi2(long long a, long long b)
	  |                   ^~~~~~~~
      CC      lib/lshrdi3.o
    lib/lshrdi3.c:9:19: warning: no previous prototype for '__lshrdi3' [-Wmissing-prototypes]
	9 | long long notrace __lshrdi3(long long u, word_type b)
	  |                   ^~~~~~~~~
      CC      lib/muldi3.o
    lib/muldi3.c:49:19: warning: no previous prototype for '__muldi3' [-Wmissing-prototypes]
       49 | long long notrace __muldi3(long long u, long long v)
	  |                   ^~~~~~~~
      CC      lib/ucmpdi2.o
    lib/ucmpdi2.c:8:19: warning: no previous prototype for '__ucmpdi2' [-Wmissing-prototypes]
	8 | word_type notrace __ucmpdi2(unsigned long long a, unsigned long long b)
	  |                   ^~~~~~~~~

Fix this by adding forward declarations to the common libgcc header
file.

Link: https://lkml.kernel.org/r/5cdbe08296693dd53849f199c3933e16e97b33c1.1682088593.git.geert+renesas@glider.be


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reported-by: kernel test robot <lkp@intel.com>
  Link: https://lore.kernel.org/oe-kbuild-all/202303272214.RxzpA6bP-lkp@intel.com/


Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>