runtime: use clock_gettime to get ns resolution for time.now & runtime.nanotime

runtime: use clock_gettime to get ns resolution for time.now & runtime.nanotime

For Linux/{386,arm}, FreeBSD/{386,amd64,arm}, NetBSD/{386,amd64}, OpenBSD/{386,amd64}.
Note: our Darwin implementation already has ns resolution.

Linux/386 (Core i7-2600 @ 3.40GHz, kernel 3.5.2-gentoo)
benchmark       old ns/op    new ns/op    delta
BenchmarkNow          110          118   +7.27%

Linux/ARM (ARM Cortex-A8 @ 800MHz, kernel 2.6.32.28 android)
benchmark       old ns/op    new ns/op    delta
BenchmarkNow          625          542  -13.28%

Linux/ARM (ARM Cortex-A9 @ 1GHz, Pandaboard)
benchmark       old ns/op    new ns/op    delta
BenchmarkNow          992          909   -8.37%

FreeBSD 9-REL-p1/amd64 (Dell R610 Server with Xeon X5650 @ 2.67GHz)
benchmark       old ns/op    new ns/op    delta
BenchmarkNow          699          695   -0.57%

FreeBSD 9-REL-p1/amd64 (Atom D525 @ 1.80GHz)
benchmark       old ns/op    new ns/op    delta
BenchmarkNow         1553         1658   +6.76%

OpenBSD/amd64 (Dell E6410 with i5 CPU M 540 @ 2.53GHz)
benchmark       old ns/op    new ns/op    delta
BenchmarkNow         1262         1236   -2.06%

OpenBSD/i386 (Asus eeePC 701 with Intel Celeron M 900MHz - locked to 631MHz)
benchmark       old ns/op    new ns/op    delta
BenchmarkNow         5089         5043   -0.90%

NetBSD/i386 (VMware VM with Core i5 CPU @ 2.7GHz)
benchmark       old ns/op    new ns/op    delta
BenchmarkNow          277          278   +0.36%

NetBSD/amd64 (VMware VM with Core i5 CPU @ 2.7Ghz)
benchmark       old ns/op    new ns/op    delta
BenchmarkNow          103          105   +1.94%

Thanks Maxim Khitrov, Joel Sing, and Dave Cheney for providing benchmark data.

R=jsing, dave, rsc
CC=golang-dev
https://golang.org/cl/6820120