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/runtime/src/instance/allocator/pooling/uffd.rs

@@ -33,6 +33,7 @@
 use super::{InstancePool, MemoryPool};
 use crate::instance::Instance;
 use anyhow::{bail, Context, Result};
+use rsix::io::{madvise, Advice};
 use std::thread;
 use userfaultfd::{Event, FeatureFlags, IoctlFlags, Uffd, UffdBuilder};
 use wasmtime_environ::{DefinedMemoryIndex, EntityRef, MemoryInitialization};
@@ -50,12 +51,7 @@ fn decommit(addr: *mut u8, len: usize) -> Result<()> {
         // and the user fault handler will receive the event.
         // If the pages are not monitored by uffd, the kernel will zero the page on next access,
         // as if it were mmap'd for the first time.
-        if libc::madvise(addr as _, len, libc::MADV_DONTNEED) != 0 {
-            bail!(
-                "madvise failed to decommit: {}",
-                std::io::Error::last_os_error()
-            );
-        }
+        madvise(addr as _, len, Advice::LinuxDontNeed).context("madvise failed to decommit")?;
     }
 
     Ok(())