runtime: use vDSO for gettimeofday on linux/amd64
runtime: use vDSO for gettimeofday on linux/amd64

Intel Core 2 Duo (2.16 GHz) running 3.6.5-1-ARCH

benchmark       old ns/op    new ns/op    delta
BenchmarkNow         1856         1034  -44.29%

R=rsc
CC=golang-dev
http://codereview.appspot.com/6826072

gnanderson/go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,tv42/old-go,tv42/old-go,dsriram/golang,apparentlymart/golang,gnanderson/go,gnanderson/go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,dsriram/golang