Remove an unnecessary Mutex around argument initialization.

In the command-line argument initialization code, remove the Mutex
around the `ARGV` and `ARGC` variables, and simply check whether
ARGV is non-null before dereferencing it. This way, if either of
ARGV or ARGC is not initialized, we'll get an empty argument list.

This allows simple cdylibs to avoid having
`pthread_mutex_lock`/`pthread_mutex_unlock` appear in their symbol
tables if they don't otherwise use threads.

Author: sunfishcode

library/std/src/sys/unix/args.rs

@@ -77,16 +77,10 @@ mod imp {
     use crate::ptr;
     use crate::sync::atomic::{AtomicIsize, AtomicPtr, Ordering};
 
-    use crate::sys_common::mutex::StaticMutex;
-
     static ARGC: AtomicIsize = AtomicIsize::new(0);
     static ARGV: AtomicPtr<*const u8> = AtomicPtr::new(ptr::null_mut());
-    // We never call `ENV_LOCK.init()`, so it is UB to attempt to
-    // acquire this mutex reentrantly!
-    static LOCK: StaticMutex = StaticMutex::new();
 
     unsafe fn really_init(argc: isize, argv: *const *const u8) {
-        let _guard = LOCK.lock();
         ARGC.store(argc, Ordering::Relaxed);
         ARGV.store(argv as *mut _, Ordering::Relaxed);
     }
@@ -128,9 +122,14 @@ mod imp {
 
     fn clone() -> Vec<OsString> {
         unsafe {
-            let _guard = LOCK.lock();
-            let argc = ARGC.load(Ordering::Relaxed);
+            // Load ARGC and ARGV without a lock. If the store to either ARGV or
+            // ARGC isn't visible yet, we'll return an empty argument list.
             let argv = ARGV.load(Ordering::Relaxed);
+            let argc = if argv.is_null() {
+                0
+            } else {
+                ARGC.load(Ordering::Relaxed)
+            };
             (0..argc)
                 .map(|i| {
                     let cstr = CStr::from_ptr(*argv.offset(i) as *const libc::c_char);