AK: Add secure_zero() implementation so it can be used on all platforms

Serenity has explicit_bzero() in LibC with the same implementation,
however we need to be able to use this from Lagom on all platforms
that we support building serenity on. I've implemented it in AK for
this reason.

AK/Memory.h

@@ -40,3 +40,16 @@ ALWAYS_INLINE void fast_u32_fill(u32* dest, u32 value, size_t count)
     }
 #endif
 }
+
+namespace AK {
+inline void secure_zero(void* ptr, size_t size)
+{
+    __builtin_memset(ptr, 0, size);
+    // The memory barrier is here to avoid the compiler optimizing
+    // away the memset when we rely on it for wiping secrets.
+    asm volatile("" ::
+                     : "memory");
+}
+}
+
+using AK::secure_zero;