x/sys/unix: Getdirentries not found on darwin arm with Go 1.11

[oxtoacart]

What version of Go are you using (go version)?

$ go version
go version go1.11.13 darwin/amd64

Does this issue reproduce with the latest release?

No

What operating system and processor architecture are you using (go env)?

go env Output

I'm using gomobile targeting darwin on arm

What did you do?

Compile a program that calls Getdirentries for darwin arm (32 bit) with Go 1.11

What did you expect to see?

Successful compilation

What did you see instead?

readdirent_getdirentries.go:18:9: undefined: Getdirentries

I think this was broken by the fix for #34400

[randall77]

@tklauser

[gopherbot]

Change https://golang.org/cl/212459 mentions this issue: unix: correct build tag to define Getdirentries for darwin/arm with Go 1.11

[oxtoacart]

@tklauser That fix LGTM.