LibC: Move Stopwatch thingy into a <serenity.h> header.

This thing is extremely useful for performance testing so let's put it here.

LibC/serenity.h

@@ -0,0 +1,34 @@
+#pragma once
+
+#include <stdio.h>
+#include <unistd.h>
+
+#ifdef __cplusplus
+
+struct Stopwatch {
+public:
+    Stopwatch(const char* name)
+        : m_name(name)
+    {
+        read_tsc(&m_start_lsw, &m_start_msw);
+    }
+
+    ~Stopwatch()
+    {
+        unsigned end_lsw;
+        unsigned end_msw;
+        read_tsc(&end_lsw, &end_msw);
+        if (m_start_msw != end_msw) {
+            dbgprintf("Stopwatch: differing msw, no result for %s\n", m_name);
+        }
+        unsigned diff = end_lsw - m_start_lsw;
+        dbgprintf("Stopwatch(%s): %u ticks\n", m_name, diff);
+    }
+
+private:
+    const char* m_name { nullptr };
+    unsigned m_start_lsw { 0 };
+    unsigned m_start_msw { 0 };
+};
+
+#endif // __cplusplus