unix: convert Darwin syscalls from raw to libSystem

Call into libSystem (Darwin's libc equivalent) to do system calls.
Raw syscalls are not a supported ABI on Darwin, they want us to use
libSystem calls instead.

The stdlib has already been converted to this new regime.  This
package is vendored into the stdlib, so we need to fix it also, then
vendor it in again.

Because this package isn't tied to a Go release, we need to keep the
old code around and use the new code only for 1.12 and up.

Update golang/go#17490

Change-Id: Idbcacff096b5bfeac871aa75dfd971570ac93322
Reviewed-on: https://go-review.googlesource.com/c/154179
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_unix_test.go

@@ -337,6 +337,12 @@ func TestRlimit(t *testing.T) {
 	}
 	set := rlimit
 	set.Cur = set.Max - 1
+	if runtime.GOOS == "darwin" && set.Cur > 10240 {
+		// The max file limit is 10240, even though
+		// the max returned by Getrlimit is 1<<63-1.
+		// This is OPEN_MAX in sys/syslimits.h.
+		set.Cur = 10240
+	}
 	err = unix.Setrlimit(unix.RLIMIT_NOFILE, &set)
 	if err != nil {
 		t.Fatalf("Setrlimit: set failed: %#v %v", set, err)