runtime: add copy of math.sqrt for use by arm softfloat

runtime: add copy of math.sqrt for use by arm softfloat

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