runtime: round spans_size up to page boundary in case we have weird (not page aligned) memory limit.
runtime: round spans_size up to page boundary
in case we have weird (not page aligned) memory limit.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/10199043

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