exp/cookiejar: implement IDNA/Punycode's toASCII.

R=dr.volker.dobler
CC=golang-dev
https://codereview.appspot.com/7398049
syscall, runtime: Plan 9: use nanotime syscall on amd64

Separates the implementation of nanotime on 64-bit
version of Plan 9 from that on the 32-bit version.
The former uses a syscall.

R=rsc, rminnich, ality
CC=golang-dev
https://codereview.appspot.com/7379051

Committer: Ron Minnich <rminnich@gmail.com>