[musl] use posix_spawn if a directory change was requested

Currently, not all libcs have the `posix_spawn_file_actions_addchdir_np` symbol
available to them. So we attempt to do a weak symbol lookup for that function.
But that only works if libc is a dynamic library -- with statically linked musl
binaries the symbol lookup would never work, so we would never be able to use it
even if the musl in use supported the symbol.

Now that Rust has a minimum musl version of 1.2.3, all supported musl versions
now include this symbol, so we can unconditionally expect it to be there. This
symbol was added to libc in rust-lang/libc#3949 -- use
it here.

I couldn't find any tests for whether the posix_spawn path is used, but I've
verified with cargo-nextest that this change works. This is a substantial
improvement to nextest's performance with musl. On my workstation with a Ryzen
7950x, against https://github.com/clap-rs/clap at
61f5ee514f8f60ed8f04c6494bdf36c19e7a8126:

Before:

```
     Summary [   1.071s] 879 tests run: 879 passed, 0 skipped
```

After:

```
     Summary [   0.392s] 879 tests run: 879 passed, 0 skipped
```

Fixes rust-lang#99740.

library/std/src/process/tests.rs

@@ -96,9 +96,23 @@ fn stdout_works() {
 #[test]
 #[cfg_attr(any(windows, target_os = "vxworks"), ignore)]
 fn set_current_dir_works() {
+    // On many Unix platforms this will use the posix_spawn path.
     let mut cmd = shell_cmd();
     cmd.arg("-c").arg("pwd").current_dir("/").stdout(Stdio::piped());
     assert_eq!(run_output(cmd), "/\n");
+
+    // Also test the fork/exec path by setting a pre_exec function.
+    #[cfg(unix)]
+    {
+        use crate::os::unix::process::CommandExt;
+
+        let mut cmd = shell_cmd();
+        cmd.arg("-c").arg("pwd").current_dir("/").stdout(Stdio::piped());
+        unsafe {
+            cmd.pre_exec(|| Ok(()));
+        }
+        assert_eq!(run_output(cmd), "/\n");
+    }
 }
 
 #[test]

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

@@ -448,7 +448,6 @@ impl Command {
         use core::sync::atomic::{AtomicU8, Ordering};
 
         use crate::mem::MaybeUninit;
-        use crate::sys::weak::weak;
         use crate::sys::{self, cvt_nz, on_broken_pipe_flag_used};
 
         if self.get_gid().is_some()
@@ -462,6 +461,8 @@ impl Command {
 
         cfg_if::cfg_if! {
             if #[cfg(target_os = "linux")] {
+                use crate::sys::weak::weak;
+
                 weak! {
                     fn pidfd_spawnp(
                         *mut libc::c_int,
@@ -575,16 +576,6 @@ impl Command {
             }
         }
 
-        // Solaris, glibc 2.29+, and musl 1.24+ can set a new working directory,
-        // and maybe others will gain this non-POSIX function too. We'll check
-        // for this weak symbol as soon as it's needed, so we can return early
-        // otherwise to do a manual chdir before exec.
-        weak! {
-            fn posix_spawn_file_actions_addchdir_np(
-                *mut libc::posix_spawn_file_actions_t,
-                *const libc::c_char
-            ) -> libc::c_int
-        }
         let addchdir = match self.get_cwd() {
             Some(cwd) => {
                 if cfg!(target_vendor = "apple") {
@@ -597,7 +588,10 @@ impl Command {
                         return Ok(None);
                     }
                 }
-                match posix_spawn_file_actions_addchdir_np.get() {
+                // Check for the availability of the posix_spawn addchdir
+                // function now. If it isn't available, bail and use the
+                // fork/exec path.
+                match get_posix_spawn_addchdir() {
                     Some(f) => Some((f, cwd)),
                     None => return Ok(None),
                 }
@@ -871,6 +865,42 @@ impl Command {
     }
 }
 
+type PosixSpawnAddChdirFn =
+    unsafe extern "C" fn(*mut libc::posix_spawn_file_actions_t, *const libc::c_char) -> libc::c_int;
+
+/// Get the function pointer for adding a chdir action to a
+/// `posix_spawn_file_actions_t`, if available, assuming a dynamic libc.
+///
+/// Some platforms can set a new working directory for a spawned process in the
+/// `posix_spawn` path. This function looks up the function pointer for adding
+/// such an action to a `posix_spawn_file_actions_t` struct.
+#[cfg(not(all(target_os = "linux", target_env = "musl")))]
+fn get_posix_spawn_addchdir() -> Option<PosixSpawnAddChdirFn> {
+    use crate::sys::weak::weak;
+
+    weak! {
+        fn posix_spawn_file_actions_addchdir_np(
+            *mut libc::posix_spawn_file_actions_t,
+            *const libc::c_char
+        ) -> libc::c_int
+    }
+
+    posix_spawn_file_actions_addchdir_np.get()
+}
+
+/// Get the function pointer for adding a chdir action to a
+/// `posix_spawn_file_actions_t`, if available, on platforms where the function
+/// is known to exist.
+///
+/// Weak symbol lookup doesn't work with statically linked libcs, so in cases
+/// where static linking is possible we need to either check for the presence
+/// of the symbol at compile time or know about it upfront.
+#[cfg(all(target_os = "linux", target_env = "musl"))]
+fn get_posix_spawn_addchdir() -> Option<PosixSpawnAddChdirFn> {
+    // Our minimum required musl supports this function, so we can just use it.
+    Some(libc::posix_spawn_file_actions_addchdir_np)
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Processes
 ////////////////////////////////////////////////////////////////////////////////