If it's not used (such as on other systems or if softfloat
is disabled) the linker will discard it.

The alternative is to teach cmd/go that every binary
depends on math implicitly on arm. I started down that
path but it's too scary. If we're going to get dependencies
right we should get dependencies right.

Fixes #6994.

LGTM=bradfitz, dave
R=golang-codereviews, bradfitz, dave
CC=golang-codereviews
https://golang.org/cl/95290043