math: use Abs in Mod rather than if x < 0 { x = -x}

goos: linux
goarch: amd64
pkg: math

name  old time/op  new time/op  delta
Mod   64.7ns ± 2%  63.7ns ± 2%  -1.52%  (p=0.003 n=8+10)

Change-Id: I851bec0fd6c223dab73e4a680b7393d49e81a0e8
Reviewed-on: https://go-review.googlesource.com/c/85095


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>