darwin: replace custom syscall package with Go native syscall package

This required a few compiler and runtime tricks to work, but I ran a
bunch of tests and it seems fine. (CI will of course do more exhaustive
testing).

The main benefit here is that we don't need to maintain the darwin
version of the syscall package, and reduce extra risks for bugs (because
we reuse the well-tested syscall package). For example, Go 1.23 needed a
bunch of new constants in the syscall package. That would have been
avoided if we had used the native syscall package on MacOS.

src/os/dir_darwin.go

@@ -135,7 +135,7 @@ func darwinOpenDir(fd syscallFd) (uintptr, string, error) {
 	}
 	var dir uintptr
 	for {
-		dir, err = syscall.Fdopendir(fd2)
+		dir, err = fdopendir(fd2)
 		if err != syscall.EINTR {
 			break
 		}
@@ -149,6 +149,9 @@ func darwinOpenDir(fd syscallFd) (uintptr, string, error) {
 
 // Implemented in syscall/syscall_libc_darwin_*.go.
 
+//go:linkname fdopendir syscall.fdopendir
+func fdopendir(fd int) (dir uintptr, err error)
+
 //go:linkname closedir syscall.closedir
 func closedir(dir uintptr) (err error)
 

src/os/file_darwin.go

@@ -0,0 +1,7 @@
+package os
+
+import "syscall"
+
+func pipe(p []int) error {
+	return syscall.Pipe(p)
+}

src/os/file_notdarwin.go

@@ -0,0 +1,9 @@
+//go:build (linux && !baremetal && !wasm_unknown) || wasip1 || wasip2
+
+package os
+
+import "syscall"
+
+func pipe(p []int) error {
+	return syscall.Pipe2(p, syscall.O_CLOEXEC)
+}

src/os/file_unix.go

@@ -70,7 +70,7 @@ func Truncate(name string, size int64) error {
 
 func Pipe() (r *File, w *File, err error) {
 	var p [2]int
-	err = handleSyscallError(syscall.Pipe2(p[:], syscall.O_CLOEXEC))
+	err = handleSyscallError(pipe(p[:]))
 	if err != nil {
 		return
 	}