ARM: 9304/1: add prototype for function called only from asm

When building with 'make W=1', the compiler warns about any function
definition that does not come with a prototype in a header, to ensure
it matches what the caller expects.

This includes functions that are only ever caller from assembly
code and don't technically need a declaration:

arch/arm/kernel/ftrace.c:227:6: error: no previous prototype for 'prepare_ftrace_return'
arch/arm/kernel/ptrace.c:850:16: error: no previous prototype for 'syscall_trace_enter'
arch/arm/kernel/ptrace.c:878:17: error: no previous prototype for 'syscall_trace_exit'
arch/arm/kernel/signal.c:601:1: error: no previous prototype for 'do_work_pending'
arch/arm/kernel/signal.c:672:17: error: no previous prototype for 'do_rseq_syscall'
arch/arm/kernel/suspend.c:75:6: error: no previous prototype for '__cpu_suspend_save'
arch/arm/kernel/traps.c:451:17: error: no previous prototype for 'do_undefinstr'
arch/arm/kernel/traps.c:516:39: error: no previous prototype for 'handle_fiq_as_nmi'
arch/arm/kernel/traps.c:535:17: error: no previous prototype for 'bad_mode'
arch/arm/kernel/traps.c:608:16: error: no previous prototype for 'arm_syscall'
arch/arm/kernel/traps.c:734:1: error: no previous prototype for 'baddataabort'
arch/arm/kernel/traps.c:774:17: error: no previous prototype for '__div0'
arch/arm/kernel/traps.c:97:6: error: no previous prototype for 'dump_backtrace_stm'
arch/arm/kernel/unwind.c:40:6: error: no previous prototype for '__aeabi_unwind_cpp_pr0'
arch/arm/kernel/unwind.c:45:6: error: no previous prototype for '__aeabi_unwind_cpp_pr1'
arch/arm/kernel/unwind.c:50:6: error: no previous prototype for '__aeabi_unwind_cpp_pr2'
arch/arm/mm/fault.c:554:1: error: no previous prototype for 'do_DataAbort'
arch/arm/mm/fault.c:584:1: error: no previous prototype for 'do_PrefetchAbort'
arch/arm/mm/proc-v7-bugs.c:280:6: error: no previous prototype for 'cpu_v7_ca8_ibe'
arch/arm/mm/proc-v7-bugs.c:293:6: error: no previous prototype for 'cpu_v7_bugs_init'
arch/arm/vdso/vgettimeofday.c:36:6: error: no previous prototype for '__aeabi_unwind_cpp_pr0'
arch/arm/vdso/vgettimeofday.c:40:6: error: no previous prototype for '__aeabi_unwind_cpp_pr1'
arch/arm/vdso/vgettimeofday.c:44:6: error: no previous prototype for '__aeabi_unwind_cpp_pr2'
arch/arm/vfp/vfpmodule.c:323:6: error: no previous prototype for 'VFP_bounce'

Add the prototypes anyway, to allow enabling this warning by default in
the future.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>