runtime: fix uint64->float32 conversion for softfloat

The fix for #48807 in CL 354429 forgot that we also need to fix
the softfloat implementation.

Update #48807

Change-Id: I596fb4e14e78145d1ad43c130b2cc5122b73655c
Reviewed-on: https://go-review.googlesource.com/c/go/+/354613


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>