math: faster Cbrt
math: faster Cbrt

For amd64, from 127 to 105 ns/op; for 386, from 208 to 169 ns/op.

R=rsc, golang-dev
CC=golang-dev
http://codereview.appspot.com/5412056

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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