[illumos] attempt to use posix_spawn to spawn processes

illumos has `posix_spawn`, and the very newest versions also have `_addchdir`,
so use that.

This is a nice ~4x performance improvement for process creation. My go-to as
usual is nextest against the clap repo, which acts as a stress test for process
creation -- with [this commit]:

```console
$ cargo nextest run -E 'not test(ui_tests) and not test(example_tests)'
before: Summary [   1.747s] 879 tests run: 879 passed, 2 skipped
after:  Summary [   0.445s] 879 tests run: 879 passed, 2 skipped
```

[this commit]: fde45f9aea

library/std/src/sys/pal/unix/process/process_unix.rs

@@ -410,6 +410,7 @@ impl Command {
 
     #[cfg(not(any(
         target_os = "freebsd",
+        target_os = "illumos",
         all(target_os = "linux", target_env = "gnu"),
         all(target_os = "linux", target_env = "musl"),
         target_os = "nto",
@@ -427,6 +428,7 @@ impl Command {
     // directly.
     #[cfg(any(
         target_os = "freebsd",
+        target_os = "illumos",
         all(target_os = "linux", target_env = "gnu"),
         all(target_os = "linux", target_env = "musl"),
         target_os = "nto",
@@ -584,6 +586,10 @@ impl Command {
         fn get_posix_spawn_addchdir() -> Option<PosixSpawnAddChdirFn> {
             use crate::sys::weak::weak;
 
+            // POSIX.1-2024 standardizes this function:
+            // https://pubs.opengroup.org/onlinepubs/9799919799/functions/posix_spawn_file_actions_addchdir.html.
+            // The _np version is more widely available, though, so try that first.
+
             weak! {
                 fn posix_spawn_file_actions_addchdir_np(
                     *mut libc::posix_spawn_file_actions_t,
@@ -591,7 +597,16 @@ impl Command {
                 ) -> libc::c_int
             }
 
-            posix_spawn_file_actions_addchdir_np.get()
+            weak! {
+                fn posix_spawn_file_actions_addchdir(
+                    *mut libc::posix_spawn_file_actions_t,
+                    *const libc::c_char
+                ) -> libc::c_int
+            }
+
+            posix_spawn_file_actions_addchdir_np
+                .get()
+                .or_else(|| posix_spawn_file_actions_addchdir.get())
         }
 
         /// Get the function pointer for adding a chdir action to a