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/unix.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 {
@@ -9,25 +9,18 @@ fn decommit(addr: *mut u8, len: usize, protect: bool) -> Result<()> {
     // mapping for the pages in the given range.
     // The new mapping will be to the CoW zero page, so this effectively
     // zeroes the pages.
-    if unsafe {
-        libc::mmap(
+    unsafe {
+        rsix::io::mmap_anonymous(
             addr as _,
             len,
             if protect {
-                libc::PROT_NONE
+                rsix::io::ProtFlags::NONE
             } else {
-                libc::PROT_READ | libc::PROT_WRITE
+                rsix::io::ProtFlags::READ | rsix::io::ProtFlags::WRITE
             },
-            libc::MAP_PRIVATE | libc::MAP_ANON | libc::MAP_FIXED,
-            -1,
-            0,
-        ) as *mut u8
-    } != addr
-    {
-        bail!(
-            "mmap failed to remap pages: {}",
-            std::io::Error::last_os_error()
-        );
+            rsix::io::MapFlags::PRIVATE | rsix::io::MapFlags::FIXED,
+        )
+        .context("mmap failed to remap pages: {}")?;
     }
 
     Ok(())