Need wasm64 syscalls

[trcrsired]

As I mentioned before, this can be done by creating separate APIs for wasm32 and wasm64.

API is 32bit
API_wasm64 is 64 bit.

The codegen from preview2 breaks my wasi-libc builds.

The size header file can switch to size_t with WASI_IMPORT macro defined.

[alexcrichton]

Thanks for the report! 64-bit memories, however, are not currently specified with the component model, so it's best to discuss on that upstream issue first. Once that's settled then code generators such as this can start growing support for 64-bit bindings.