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/linux.rs

@@ -1,4 +1,4 @@
-use anyhow::{bail, Context, Result};
+use anyhow::{Context, Result};
 
 fn decommit(addr: *mut u8, len: usize, protect: bool) -> Result<()> {
     if len == 0 {
@@ -12,12 +12,8 @@ fn decommit(addr: *mut u8, len: usize, protect: bool) -> Result<()> {
         }
 
         // On Linux, this is enough to cause the kernel to initialize the pages to 0 on next access
-        if libc::madvise(addr as _, len, libc::MADV_DONTNEED) != 0 {
-            bail!(
-                "madvise failed to decommit: {}",
-                std::io::Error::last_os_error()
-            );
-        }
+        rsix::io::madvise(addr as _, len, rsix::io::Advice::LinuxDontNeed)
+            .context("madvise failed to decommit: {}")?;
     }
 
     Ok(())