cmd/compile: emit fused multiply-{add,subtract} on ppc64x

cmd/compile: emit fused multiply-{add,subtract} on ppc64x

A follow on to CL 36963 adding support for ppc64x.

Performance changes (as posted on the issue):

poly1305:
benchmark               old ns/op new ns/op delta
Benchmark64-16          172       151       -12.21%
Benchmark1K-16          1828      1523      -16.68%
Benchmark64Unaligned-16 172       151       -12.21%
Benchmark1KUnaligned-16 1827      1523      -16.64%

math:
BenchmarkAcos-16        43.9      39.9      -9.11%
BenchmarkAcosh-16       57.0      45.8      -19.65%
BenchmarkAsin-16        35.8      33.0      -7.82%
BenchmarkAsinh-16       68.6      60.8      -11.37%
BenchmarkAtan-16        19.8      16.2      -18.18%
BenchmarkAtanh-16       65.5      57.5      -12.21%
BenchmarkAtan2-16       45.4      34.2      -24.67%
BenchmarkGamma-16       37.6      26.0      -30.85%
BenchmarkLgamma-16      40.0      28.2      -29.50%
BenchmarkLog1p-16       35.1      29.1      -17.09%
BenchmarkSin-16         22.7      18.4      -18.94%
BenchmarkSincos-16      31.7      23.7      -25.24%
BenchmarkSinh-16        146       131       -10.27%
BenchmarkY0-16          130       107       -17.69%
BenchmarkY1-16          127       107       -15.75%
BenchmarkYn-16          278       235       -15.47%

Updates #17895.

Change-Id: I1c16199715d20c9c4bd97c4a950bcfa69eb688c1
Reviewed-on: https://go-review.googlesource.com/38095
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>