If a program wants to evaluate math.Sqrt for any constant value
(for example, math.Sqrt(3)), we can replace that expression with
its evaluation (1.7320508075688772) at compile time, instead of
generating a SQRT assembly command or equivalent.

Adds tests that math.Sqrt generates the correct values. I also
compiled a short program and verified that the Sqrt expression was
replaced by a constant value in the "after opt" step.

Adds a short doc to the top of generic.rules explaining what the file
does and how other files interact with it.

Fixes #15543.

Change-Id: I6b6e63ac61cec50763a09ba581024adeee03d4fa
Reviewed-on: https://go-review.googlesource.com/27457Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>