cmd/compile: lower float to uint conversions on s390x

cmd/compile: lower float to uint conversions on s390x

Add rules for lowering float <-> unsigned int on s390x.

During compilation,
Cvt64Uto64F rule triggers around 80 times,
Cvt64Fto64U rule triggers around 20 times,
Cvt64Uto32F rule triggers around 5 times.

Change-Id: If4c9d128b9132fce8c0bea9abc09cb43a5df7989
Reviewed-on: https://go-review.googlesource.com/c/go/+/209177
Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>