math, cmd/internal/obj/ppc64: improve floor, ceil, trunc with asm

math, cmd/internal/obj/ppc64: improve floor, ceil, trunc with asm

This adds the instructions frim, frip, and friz to the ppc64x
assembler for use in implementing the math.Floor, math.Ceil, and
math.Trunc functions to improve performance.

Fixes #17185

BenchmarkCeil-128                    21.4          6.99          -67.34%
BenchmarkFloor-128                   13.9          6.37          -54.17%
BenchmarkTrunc-128                   12.7          6.33          -50.16%

Change-Id: I96131bd4e8c9c8dbafb25bfeb544cf9d2dbb4282
Reviewed-on: https://go-review.googlesource.com/29654
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Michael Munday <munday@ca.ibm.com>