Kernel: Mark compilation-unit-only functions as static

This enables a nice warning in case a function becomes dead code. Also, in case
of signal_trampoline_dummy, marking it external (non-static) prevents it from
being 'optimized away', which would lead to surprising and weird linker errors.

I found these places by using -Wmissing-declarations.

The Kernel still shows these issues, which I think are false-positives,
but don't want to touch:
- Kernel/Arch/i386/CPU.cpp:1081:17: void Kernel::enter_thread_context(Kernel::Thread*, Kernel::Thread*)
- Kernel/Arch/i386/CPU.cpp:1170:17: void Kernel::context_first_init(Kernel::Thread*, Kernel::Thread*, Kernel::TrapFrame*)
- Kernel/Arch/i386/CPU.cpp:1304:16: u32 Kernel::do_init_context(Kernel::Thread*, u32)
- Kernel/Arch/i386/CPU.cpp:1347:17: void Kernel::pre_init_finished()
- Kernel/Arch/i386/CPU.cpp:1360:17: void Kernel::post_init_finished()
	No idea, not gonna touch it.
- Kernel/init.cpp:104:30: void Kernel::init()
- Kernel/init.cpp:167:30: void Kernel::init_ap(u32, Kernel::Processor*)
- Kernel/init.cpp:184:17: void Kernel::init_finished(u32)
	Called by boot.S.
- Kernel/init.cpp:383:16: int Kernel::__cxa_atexit(void (*)(void*), void*, void*)
- Kernel/StdLib.cpp:285:19: void __cxa_pure_virtual()
- Kernel/StdLib.cpp:300:19: void __stack_chk_fail()
- Kernel/StdLib.cpp:305:19: void __stack_chk_fail_local()
	Not sure how to tell the compiler that the compiler is already using them.
	Also, maybe __cxa_atexit should go into StdLib.cpp?
- Kernel/Modules/TestModule.cpp:31:17: void module_init()
- Kernel/Modules/TestModule.cpp:40:17: void module_fini()
	Could maybe go into a new header. This would also provide type-checking for new modules.

Kernel/Thread.cpp

@@ -359,7 +359,7 @@ enum class DefaultSignalAction {
     Continue,
 };
 
-DefaultSignalAction default_signal_action(u8 signal)
+static DefaultSignalAction default_signal_action(u8 signal)
 {
     ASSERT(signal && signal < NSIG);