arch: add do_page_fault prototypes

do_page_fault() is missing a declaration on a couple of architectures:

arch/alpha/mm/fault.c:85:1: error: no previous prototype for 'do_page_fault' [-Werror=missing-prototypes]
arch/csky/mm/fault.c:187:17: error: no previous prototype for 'do_page_fault' [-Werror=missing-prototypes]
arch/mips/mm/fault.c:323:17: error: no previous prototype for 'do_page_fault' [-Werror=missing-prototypes]
arch/nios2/mm/fault.c:43:17: error: no previous prototype for 'do_page_fault' [-Werror=missing-prototypes]
arch/sh/mm/fault.c:389:27: error: no previous prototype for 'do_page_fault' [-Werror=missing-prototypes]

Since the calling conventions are architecture specific here,
add separate prototypes for each one.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>