rts: fix pointer overflow undefined behavior in bytecode interpreter

This patch fixes an unnoticed undefined behavior in the bytecode
interpreter. It can be caught by building `rts/Interpreter.c` with
`-fsanitize=pointer-overflow`, the warning message is something like:

```
rts/Interpreter.c:1369:13: runtime error: addition of unsigned offset to 0x004200197660 overflowed to 0x004200197658
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior rts/Interpreter.c:1369:13
rts/Interpreter.c:1265:13: runtime error: addition of unsigned offset to 0x004200197660 overflowed to 0x004200197658
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior rts/Interpreter.c:1265:13
rts/Interpreter.c:1645:13: runtime error: addition of unsigned offset to 0x0042000b22f8 overflowed to 0x0042000b22f0
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior rts/Interpreter.c:1645:13
```

Whenever we do something like `SpW(-1)`, the negative argument is
implicitly converted to an unsigned integer type and causes pointer
arithmetic overflow. It happens to be harmless for most targets since
overflowing would wrap the result to desired value, but it's still
coincidental and undefined behavior. Furthermore, it causes real
damage to the wasm backend, given clang-20 will emit invalid wasm code
that crashes at run-time for this kind of C code! (see
https://github.com/llvm/llvm-project/issues/108770)

The fix here is adding some explicit casts to ensure we always use the
signed `ptrdiff_t` type as right hand operand of pointer arithmetic.