cmd/compile: intrinsics for trunc, floor, ceil on ppc64x

cmd/compile: intrinsics for trunc, floor, ceil on ppc64x

This implements trunc, floor, and ceil in the math package
as intrinsics on ppc64x.  Significant improvement mainly due
to avoiding call overhead of args and return value.

BenchmarkCeil-16                    5.95          0.69          -88.40%
BenchmarkFloor-16                   5.95          0.69          -88.40%
BenchmarkTrunc-16                   5.82          0.69          -88.14%

Updates #21390

Change-Id: I951e182694f6e0c431da79c577272b81fb0ebad0
Reviewed-on: https://go-review.googlesource.com/54654
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: David Chase <drchase@google.com>