Update poll_oneoff API

This commit updates `poll_oneoff`'s API in a potentially least
invasive way. That is, it adds unused `WasiCtx` argument to the
syscall which will be required by #137. I am hopeful that this way
 #137 can pass all tests and hence this commit should aid the review
 process.

src/hostcalls/misc.rs

@@ -72,6 +72,7 @@ hostcalls! {
     ) -> wasm32::__wasi_errno_t;
 
     pub unsafe fn poll_oneoff(
+        wasi_ctx: &WasiCtx,
         memory: &mut [u8],
         input: wasm32::uintptr_t,
         output: wasm32::uintptr_t,

src/hostcalls_impl/misc.rs

@@ -183,6 +183,7 @@ pub(crate) fn clock_time_get(
 }
 
 pub(crate) fn poll_oneoff(
+    _wasi_ctx: &WasiCtx,
     memory: &mut [u8],
     input: wasm32::uintptr_t,
     output: wasm32::uintptr_t,