This change improves the performance of memmove
on ppc64 & ppc64le mainly for moves >=32 bytes.
In addition, the test to detect backward moves
 was enhanced to avoid backward moves if source
and dest were in different types of storage, since
backward moves might not always be efficient.

Fixes #14507

The following shows some of the improvements from the test
in the runtime package:

BenchmarkMemmove32                   4229.56      4717.13      1.12x
BenchmarkMemmove64                   6156.03      7810.42      1.27x
BenchmarkMemmove128                  7521.69      12468.54     1.66x
BenchmarkMemmove256                  6729.90      18260.33     2.71x
BenchmarkMemmove512                  8521.59      18033.81     2.12x
BenchmarkMemmove1024                 9760.92      25762.61     2.64x
BenchmarkMemmove2048                 10241.00     29584.94     2.89x
BenchmarkMemmove4096                 10399.37     31882.31     3.07x

BenchmarkMemmoveUnalignedDst16       1943.69      2258.33      1.16x
BenchmarkMemmoveUnalignedDst32       3885.08      3965.81      1.02x
BenchmarkMemmoveUnalignedDst64       5121.63      6965.54      1.36x
BenchmarkMemmoveUnalignedDst128      7212.34      11372.68     1.58x
BenchmarkMemmoveUnalignedDst256      6564.52      16913.59     2.58x
BenchmarkMemmoveUnalignedDst512      8364.35      17782.57     2.13x
BenchmarkMemmoveUnalignedDst1024     9539.87      24914.72     2.61x
BenchmarkMemmoveUnalignedDst2048     9199.23      21235.11     2.31x
BenchmarkMemmoveUnalignedDst4096     10077.39     25231.99     2.50x

BenchmarkMemmoveUnalignedSrc32       3249.83      3742.52      1.15x
BenchmarkMemmoveUnalignedSrc64       5562.35      6627.96      1.19x
BenchmarkMemmoveUnalignedSrc128      6023.98      10200.84     1.69x
BenchmarkMemmoveUnalignedSrc256      6921.83      15258.43     2.20x
BenchmarkMemmoveUnalignedSrc512      8593.13      16541.97     1.93x
BenchmarkMemmoveUnalignedSrc1024     9730.95      22927.84     2.36x
BenchmarkMemmoveUnalignedSrc2048     9793.28      21537.73     2.20x
BenchmarkMemmoveUnalignedSrc4096     10132.96     26295.06     2.60x

Change-Id: I73af59970d4c97c728deabb9708b31ec7e01bdf2
Reviewed-on: https://go-review.googlesource.com/21990Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>