math: replace assembly implementations of Abs with pure Go version

math: replace assembly implementations of Abs with pure Go version

The compiler can do a fine job, and can also inline it.

From Jeremy Jackins's observation and rsc's recommendation in thread:

"Pure Go math.Abs outperforms assembly version"
https://groups.google.com/forum/#!topic/golang-dev/nP5mWvwAXZo

Updates #13095

Change-Id: I3066f8eaa327bb403173b29791cc8661d7c0532c
Reviewed-on: https://go-review.googlesource.com/16444
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>