runtime: use MOVSB instead of MOVSQ for unaligned moves

runtime: use MOVSB instead of MOVSQ for unaligned moves

MOVSB is quite a bit faster for unaligned moves.
Possibly we should use MOVSB all of the time, but Intel folks
say it might be a bit faster to use MOVSQ on some processors
(but not any I have access to at the moment).

benchmark                              old ns/op     new ns/op     delta
BenchmarkMemmove4096-8                 93.9          93.2          -0.75%
BenchmarkMemmoveUnalignedDst4096-8     256           151           -41.02%
BenchmarkMemmoveUnalignedSrc4096-8     175           90.5          -48.29%

Fixes #14630

Change-Id: I568e6d6590eb3615e6a699fb474020596be665ff
Reviewed-on: https://go-review.googlesource.com/20293
Reviewed-by: Ian Lance Taylor <iant@golang.org>