runtime: memclr perf improvements on ppc64x

runtime: memclr perf improvements on ppc64x

This updates runtime/memclr_ppc64x.s to improve performance,
by unrolling loops for larger clears.

Fixes #17348

benchmark                    old MB/s     new MB/s     speedup
BenchmarkMemclr/5-80         199.71       406.63       2.04x
BenchmarkMemclr/16-80        693.66       1817.41      2.62x
BenchmarkMemclr/64-80        2309.35      5793.34      2.51x
BenchmarkMemclr/256-80       5428.18      14765.81     2.72x
BenchmarkMemclr/4096-80      8611.65      27191.94     3.16x
BenchmarkMemclr/65536-80     8736.69      28604.23     3.27x
BenchmarkMemclr/1M-80        9304.94      27600.09     2.97x
BenchmarkMemclr/4M-80        8705.66      27589.64     3.17x
BenchmarkMemclr/8M-80        8575.74      23631.04     2.76x
BenchmarkMemclr/16M-80       8443.10      19240.68     2.28x
BenchmarkMemclr/64M-80       8390.40      9493.04      1.13x
BenchmarkGoMemclr/5-80       263.05       630.37       2.40x
BenchmarkGoMemclr/16-80      904.33       1148.49      1.27x
BenchmarkGoMemclr/64-80      2830.20      8756.70      3.09x
BenchmarkGoMemclr/256-80     6064.59      20299.46     3.35x

Change-Id: Ic76c9183c8b4129ba3df512ca8b0fe6bd424e088
Reviewed-on: https://go-review.googlesource.com/30373
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <munday@ca.ibm.com>
Reviewed-by: David Chase <drchase@google.com>