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`.

crates/fiber/Cargo.toml

@@ -14,7 +14,7 @@ edition = "2018"
 links = "wasmtime-fiber-shims"
 
 [target.'cfg(unix)'.dependencies]
-libc = "0.2.80"
+rsix = "0.23.0"
 
 [target.'cfg(windows)'.dependencies.winapi]
 version = "0.3.9"