Revert "os: set TMPDIR on darwin/arm"

This reverts commit 87a0d39.

Looks like introducing file_darwin_arm.go is confusing something in the API checker (probably go/types) into ignoring file.go, so the O_SYNC symbol is being lost.

No actual bug in this CL AFAIK, but I'll fix the other bug later and then roll this forward.

Change-Id: Ic132fb101e4b5f2716f7a0d15872bf35bdf42139
Reviewed-on: https://go-review.googlesource.com/6331
Reviewed-by: David Crawshaw <crawshaw@golang.org>