UserspaceEmulator: Catch use-after-frees by tracking malloc/free :^)

This patch introduces a "MallocTracer" to the UserspaceEmulator.
If this object is present on the Emulator, it can be notified whenever
the emulated program does a malloc() or free().

The notifications come in via a magic instruction sequence that we
embed in the LibC malloc() and free() functions. The sequence is:

    "salc x2, push reg32 x2, pop reg32 x3"

The data about the malloc/free operation is in the three pushes.
We make sure the sequence is harmless when running natively.

Memory accesses on MmapRegion are then audited to see if they fall
inside a known-to-be-freed malloc chunk. If so, we complain loud
and red in the debugger output. :^)

This is very, very cool! :^)

It's also a whole lot slower than before, since now we're auditing
memory accesses against a new set of metadata. This will need to be
optimized (and running in this mode should be opt-in, perhaps even
a separate program, etc.)

DevTools/UserspaceEmulator/SoftCPU.h

@@ -799,6 +799,8 @@ private:
 
     void update_code_cache();
 
+    void did_receive_secret_data();
+
 private:
     Emulator& m_emulator;
 
@@ -810,6 +812,9 @@ private:
 
     const u8* m_cached_code_ptr { nullptr };
     const u8* m_cached_code_end { nullptr };
+
+    u32 m_secret_handshake_state { 0 };
+    u32 m_secret_data[3];
 };
 
 ALWAYS_INLINE u8 SoftCPU::read8()