syscall: accept pre-existing directories in nacl zip file

NaCl creates /tmp. This lets the zip file populate it.

LGTM=adg
R=adg
CC=golang-codereviews
https://codereview.appspot.com/159600043
runtime: disable fake time on nacl

This leaked into the CL I submitted for Minux,
because I was testing it.

TBR=adg
CC=golang-codereviews
https://codereview.appspot.com/159600044