cmd/internal/obj/ppc64: generate big uint32 values in register

cmd/internal/obj/ppc64: generate big uint32 values in register

When using "MOVD $const, Rx", any 32b constant can be generated in
register quickly. Avoid transforming big uint32 values into a load.

And, fix the instance in runtime.usleep where I discovered this.

Change-Id: I46e156d7edf200f85b5b61162f00223c0ad81fe2
Reviewed-on: https://go-review.googlesource.com/c/go/+/444815
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>