math: replace float32/64 extrema with exact expressions

Follow-up on https://golang.org/cl/315170.

Updates #44057.
Updates #44058.

Change-Id: I0b071e8ee7a1c97aae2436945cc9583cde3b40b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/315969


Trust: Robert Griesemer <gri@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>