unix: remove Getdirentries on iOS

This system call doesn't exist on iOS.

Update golang/go#28984

Change-Id: I92eb6fd3eb263863a31338bc18c9826a2242434b
Reviewed-on: https://go-review.googlesource.com/c/154659
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/unix/syscall_darwin_arm.go b/unix/syscall_darwin_arm.go
index fbe381a..0bc2f38 100644
--- a/unix/syscall_darwin_arm.go
+++ b/unix/syscall_darwin_arm.go
@@ -71,3 +71,7 @@
 //sys	Lstat(path string, stat *Stat_t) (err error)
 //sys	Stat(path string, stat *Stat_t) (err error)
 //sys	Statfs(path string, stat *Statfs_t) (err error)
+
+func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
+	return 0, ENOSYS
+}