unix: avoid __getdirentries64 on darwin

Getdirentries is implemented with the __getdirentries64 function
in libSystem.dylib on darwin/{386,amd64}. That function can't be used in
an app store application.

Implement Getdirentries using the underlying
fdopendir/readdir_r/closedir for Go 1.13. The simulation isn't faithful,
and could be slow, but it should handle common cases.

For Go 1.12, fall back to raw syscalls since syscall.syscallPtr needed
to use fdopendir from libSystem.dylib is not available.

Follow CL 168479 and CL 170892 which did the same for syscall in the
stdlib.

Tested on darwin/amd64 with Go 1.11, Go 1.12 and Go 1.13

Fixes golang/go#34400

Change-Id: I631382aaea9ee7e0c4ed09e06ad5427efc620769
Reviewed-on: https://go-review.googlesource.com/c/sys/+/196478
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

unix/mksyscall.go

@@ -121,7 +121,7 @@ func main() {
 	}
 
 	libc := false
-	if goos == "darwin" && strings.Contains(buildTags(), ",go1.12") {
+	if goos == "darwin" && (strings.Contains(buildTags(), ",go1.12") || strings.Contains(buildTags(), ",go1.13")) {
 		libc = true
 	}
 	trampolines := map[string]bool{}
@@ -292,11 +292,6 @@ func main() {
 				asm = "syscall_" + strings.ToLower(asm[:1]) + asm[1:] // internal syscall call
 				sysname = strings.TrimPrefix(sysname, "SYS_")         // remove SYS_
 				sysname = strings.ToLower(sysname)                    // lowercase
-				if sysname == "getdirentries64" {
-					// Special case - libSystem name and
-					// raw syscall name don't match.
-					sysname = "__getdirentries64"
-				}
 				libcFn = sysname
 				sysname = "funcPC(libc_" + sysname + "_trampoline)"
 			}