math: use shared signMask constant

math: use shared signMask constant

In abs.go and copysign.go, magic numbers and local constants are
used for the sign bit mask (1 << 63), even though a shared constant
signMask already exists in bits.go.

Change-Id: Ic3aeb9b52674538443cbe074acfeb373a3c74a8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/732060
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>