cmd/nm: add -sort=size

cmd/nm: add -sort=size
When printing the size, we often want to sort on that key.
Because it's used when looking for large things, make the
sort go from largest to smallest.

Perfect recreation of CL 45150044, which was lost to some blunder.

R=golang-codereviews, gobot, rsc
CC=golang-codereviews
https://golang.org/cl/48500044