Use rsix to make system calls in Wasmtime. (#3355)

* Use rsix to make system calls in Wasmtime.

`rsix` is a system call wrapper crate that we use in `wasi-common`,
which can provide the following advantages in the rest of Wasmtime:

 - It eliminates some `unsafe` blocks in Wasmtime's code. There's
   still an `unsafe` block in the library, but this way, the `unsafe`
   is factored out and clearly scoped.

 - And, it makes error handling more consistent, factoring out code for
   checking return values and `io::Error::last_os_error()`, and code that
   does `errno::set_errno(0)`.

This doesn't cover *all* system calls; `rsix` doesn't implement
signal-handling APIs, and this doesn't cover calls made through `std` or
crates like `userfaultfd`, `rand`, and `region`.

.github/workflows/main.yml

@@ -163,6 +163,7 @@ jobs:
     - run: |
         rustup target add wasm32-unknown-emscripten
         rustup target add armv7-unknown-linux-gnueabihf
+        sudo apt-get install -y gcc-arm-linux-gnueabihf
     - run: cargo check --target wasm32-unknown-emscripten -p wasi-common
     - run: cargo check --target armv7-unknown-linux-gnueabihf -p wasi-common