unix: fix build with Go 1.12 on darwin
unix: fix build with Go 1.12 on darwin

CL 250437 broke the build with Go 1.12 on darwin, due to
SYS_GETDIRENTRIES64 being removed. However, this is still needed to
implement Getdirentries for Go 1.12 because it does not provide
syscall_syscallPtr for fdopendir. Thus, add _SYS_GETDIRENTRIES64 as an
unexported const.

Change-Id: I52b420095deeb50d7fac002702dfce01cbf34bb7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/251317
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys