cmd/compile: don't store NaN in ppc64 floating point constant ops

Missed in CL 221790

This is the only remaining use of math.Float64frombits in the .rules
file that isn't already guarded.

Fixes #38880

Change-Id: I11f71e3a48516748d8d2701c6cf6920a7bc9e216
Reviewed-on: https://go-review.googlesource.com/c/go/+/232859


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>