Sprinkle const around where appropriate (#1909)

* Sprinkle `const` around where appropriate

- This will make it easier to use `NDS` objects in `const` contexts (e.g. `const` parameters or methods)

* Remove the `const` qualifier on `DSi_DSP::DSPRead16`

- MMIO reads can be non-pure, so this may not be `const` in the future

src/ARMJIT_A64/ARMJIT_Compiler.h

@@ -114,7 +114,7 @@ public:
 
     bool CanCompile(bool thumb, u16 kind);
 
-    bool FlagsNZNeeded()
+    bool FlagsNZNeeded() const
     {
         return CurInstr.SetFlags & 0xC;
     }
@@ -234,7 +234,7 @@ public:
         return (u8*)entry - GetRXBase();
     }
 
-    bool IsJITFault(u8* pc);
+    bool IsJITFault(const u8* pc);
     u8* RewriteMemAccess(u8* pc);
 
     void SwapCodeRegion()