cmd/compile: use power5 instructions for uint64 to float casts

cmd/compile: use power5 instructions for uint64 to float casts

Use the same technique as mips64 for these casts (CL 22835).

We could use the FCFIDU instruction for ppc64le however it seems
better to keep it the same as ppc64 for now.

Updates #15539, updates #16004.

Change-Id: I550680e485327568bf3238c4615a6cc8de6438d7
Reviewed-on: https://go-review.googlesource.com/24191
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>