math: fix Abs, Copysign and Signbit benchmarks

CL 62250 makes constant folding a bit more aggressive and these
benchmarks were optimized away. This CL adds some indirection to
the function arguments to stop them being folded.

The Copysign benchmark is a bit faster because I've left one
argument as a constant and it can be partially folded.

                     old           CL 62250     this CL
Copysign             1.24ns ± 0%   0.34ns ± 2%  1.02ns ± 2%
Abs                  0.67ns ± 0%   0.35ns ± 3%  0.67ns ± 0%
Signbit              0.87ns ± 0%   0.35ns ± 2%  0.87ns ± 1%

Change-Id: I9604465a87d7aa29f4bd6009839c8ee354be3cd7
Reviewed-on: https://go-review.googlesource.com/62450


Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>