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>