[release-branch.go1.5] runtime: adjust the ppc64x memmove and memclr to copy by word...

[release-branch.go1.5] runtime: adjust the ppc64x memmove and memclr to copy by word as much as it can

Issue #12552 can happen on ppc64 too, although much less frequently in my
testing. I'm fairly sure this fixes it (2 out of 200 runs of oracle.test failed
without this change and 0 of 200 failed with it). It's also a lot faster for
large moves/clears:

name           old speed      new speed       delta
Memmove1-6      157MB/s ± 9%    144MB/s ± 0%    -8.20%         (p=0.004 n=10+9)
Memmove2-6      281MB/s ± 1%    249MB/s ± 1%   -11.53%        (p=0.000 n=10+10)
Memmove3-6      376MB/s ± 1%    328MB/s ± 1%   -12.64%        (p=0.000 n=10+10)
Memmove4-6      475MB/s ± 4%    345MB/s ± 1%   -27.28%         (p=0.000 n=10+8)
Memmove5-6      540MB/s ± 1%    393MB/s ± 0%   -27.21%        (p=0.000 n=10+10)
Memmove6-6      609MB/s ± 0%    423MB/s ± 0%   -30.56%         (p=0.000 n=9+10)
Memmove7-6      659MB/s ± 0%    468MB/s ± 0%   -28.99%         (p=0.000 n=8+10)
Memmove8-6      705MB/s ± 0%   1295MB/s ± 1%   +83.73%          (p=0.000 n=9+9)
Memmove9-6      740MB/s ± 1%   1241MB/s ± 1%   +67.61%         (p=0.000 n=10+8)
Memmove10-6     780MB/s ± 0%   1162MB/s ± 1%   +48.95%         (p=0.000 n=10+9)
Memmove11-6     811MB/s ± 0%   1180MB/s ± 0%   +45.58%          (p=0.000 n=8+9)
Memmove12-6     820MB/s ± 1%   1073MB/s ± 1%   +30.83%         (p=0.000 n=10+9)
Memmove13-6     849MB/s ± 0%   1068MB/s ± 1%   +25.87%        (p=0.000 n=10+10)
Memmove14-6     877MB/s ± 0%    911MB/s ± 0%    +3.83%        (p=0.000 n=10+10)
Memmove15-6     893MB/s ± 0%    922MB/s ± 0%    +3.25%         (p=0.000 n=10+9)
Memmove16-6     897MB/s ± 1%   2418MB/s ± 1%  +169.67%         (p=0.000 n=10+9)
Memmove32-6     908MB/s ± 0%   3927MB/s ± 2%  +332.64%         (p=0.000 n=10+8)
Memmove64-6    1.11GB/s ± 0%   5.59GB/s ± 0%  +404.64%          (p=0.000 n=9+9)
Memmove128-6   1.25GB/s ± 0%   6.71GB/s ± 2%  +437.49%         (p=0.000 n=9+10)
Memmove256-6   1.33GB/s ± 0%   7.25GB/s ± 1%  +445.06%        (p=0.000 n=10+10)
Memmove512-6   1.38GB/s ± 0%   8.87GB/s ± 0%  +544.43%        (p=0.000 n=10+10)
Memmove1024-6  1.40GB/s ± 0%  10.00GB/s ± 0%  +613.80%        (p=0.000 n=10+10)
Memmove2048-6  1.41GB/s ± 0%  10.65GB/s ± 0%  +652.95%         (p=0.000 n=9+10)
Memmove4096-6  1.42GB/s ± 0%  11.01GB/s ± 0%  +675.37%         (p=0.000 n=8+10)
Memclr5-6       269MB/s ± 1%    264MB/s ± 0%    -1.80%        (p=0.000 n=10+10)
Memclr16-6      600MB/s ± 0%    887MB/s ± 1%   +47.83%        (p=0.000 n=10+10)
Memclr64-6     1.06GB/s ± 0%   2.91GB/s ± 1%  +174.58%         (p=0.000 n=8+10)
Memclr256-6    1.32GB/s ± 0%   6.58GB/s ± 0%  +399.86%         (p=0.000 n=9+10)
Memclr4096-6   1.42GB/s ± 0%  10.90GB/s ± 0%  +668.03%         (p=0.000 n=8+10)
Memclr65536-6  1.43GB/s ± 0%  11.37GB/s ± 0%  +697.83%          (p=0.000 n=9+8)
GoMemclr5-6     359MB/s ± 0%    360MB/s ± 0%    +0.46%        (p=0.000 n=10+10)
GoMemclr16-6    750MB/s ± 0%   1264MB/s ± 1%   +68.45%        (p=0.000 n=10+10)
GoMemclr64-6   1.17GB/s ± 0%   3.78GB/s ± 1%  +223.58%         (p=0.000 n=10+9)
GoMemclr256-6  1.35GB/s ± 0%   7.47GB/s ± 0%  +452.44%        (p=0.000 n=10+10)

Update #12552

Change-Id: I7192e9deb9684a843aed37f58a16a4e29970e893
Reviewed-on: https://go-review.googlesource.com/14840
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/16907
Reviewed-by: Russ Cox <rsc@golang.org>