Performance improvement aside, time.Now() now gets real nanosecond resolution
on supported systems.

Benchmark done on Core i7-2600 @ 3.40GHz with kernel 3.5.2-gentoo.
original vDSO gettimeofday:
BenchmarkNow    100000000               27.4 ns/op
new vDSO gettimeofday fallback:
BenchmarkNow    100000000               27.6 ns/op
new vDSO clock_gettime:
BenchmarkNow    100000000               24.4 ns/op

R=golang-dev, bradfitz, iant, iant
CC=golang-dev
https://golang.org/cl/6814103