AK+Kernel+LibELF: Remove the need for IteratorDecision::Continue

By constraining two implementations, the compiler will select the best
fitting one. All this will require is duplicating the implementation and
simplifying for the `void` case.

This constraining also informs both the caller and compiler by passing
the callback parameter types as part of the constraint
(e.g.: `IterationFunction<int>`).

Some `for_each` functions in LibELF only take functions which return
`void`. This is a minimal correctness check, as it removes one way for a
function to incompletely do something.

There seems to be a possible idiom where inside a lambda, a `return;` is
the same as `continue;` in a for-loop.

Kernel/Arch/i386/CPU.cpp

@@ -2090,12 +2090,11 @@ void Processor::smp_broadcast_message(ProcessorMessage& msg)
     VERIFY(msg.refs > 0);
     bool need_broadcast = false;
     for_each(
-        [&](Processor& proc) -> IterationDecision {
+        [&](Processor& proc) {
             if (&proc != &cur_proc) {
                 if (proc.smp_queue_message(msg))
                     need_broadcast = true;
             }
-            return IterationDecision::Continue;
         });
 
     // Now trigger an IPI on all other APs (unless all targets already had messages queued)
@@ -2215,9 +2214,8 @@ void Processor::smp_broadcast_halt()
     // We don't want to use a message, because this could have been triggered
     // by being out of memory and we might not be able to get a message
     for_each(
-        [&](Processor& proc) -> IterationDecision {
+        [&](Processor& proc) {
             proc.m_halt_requested.store(true, AK::MemoryOrder::memory_order_release);
-            return IterationDecision::Continue;
         });
 
     // Now trigger an IPI on all other APs

Kernel/Arch/x86/CPU.h

@@ -8,6 +8,7 @@
 
 #include <AK/Atomic.h>
 #include <AK/Badge.h>
+#include <AK/Concepts.h>
 #include <AK/Noncopyable.h>
 #include <AK/Vector.h>
 
@@ -733,7 +734,7 @@ public:
 
     static size_t processor_count() { return processors().size(); }
 
-    template<typename Callback>
+    template<IteratorFunction<Processor&> Callback>
     static inline IterationDecision for_each(Callback callback)
     {
         auto& procs = processors();
@@ -745,6 +746,16 @@ public:
         return IterationDecision::Continue;
     }
 
+    template<VoidFunction<Processor&> Callback>
+    static inline IterationDecision for_each(Callback callback)
+    {
+        auto& procs = processors();
+        size_t count = procs.size();
+        for (size_t i = 0; i < count; i++)
+            callback(*procs[i]);
+        return IterationDecision::Continue;
+    }
+
     ALWAYS_INLINE u8 physical_address_bit_width() const { return m_physical_address_bit_width; }
 
     ALWAYS_INLINE ProcessorInfo& info() { return *m_info; }