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`.

cranelift/native/src/lib.rs

@@ -126,7 +126,7 @@ pub fn builder_with_options(
     }
 
     // There is no is_s390x_feature_detected macro yet, so for now
-    // we use getauxval from the libc crate directly.
+    // we use linux_hwcap from the rsix crate directly.
     #[cfg(all(target_arch = "s390x", target_os = "linux"))]
     {
         use cranelift_codegen::settings::Configurable;
@@ -135,8 +135,8 @@ pub fn builder_with_options(
             return Ok(isa_builder);
         }
 
-        let v = unsafe { libc::getauxval(libc::AT_HWCAP) };
-        const HWCAP_S390X_VXRS_EXT2: libc::c_ulong = 32768;
+        let v = rsix::process::linux_hwcap().0;
+        const HWCAP_S390X_VXRS_EXT2: usize = 32768;
         if (v & HWCAP_S390X_VXRS_EXT2) != 0 {
             isa_builder.enable("has_vxrs_ext2").unwrap();
             // There is no separate HWCAP bit for mie2, so assume