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.