runtime: use vDSO clock_gettime for time.now & runtime.nanotime on Linux/amd64
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