runtime: use unoptimized memmove and memclr on Plan 9
On Plan 9, the kernel disallows the use of floating point
instructions while handling a note. Previously, we worked
around this by using a simple loop in place of memmove.
When I added that work-around, I verified that all paths
from the note handler didn't end up calling memmove. Now
that memclr is using SSE instructions, the same process
will have to be done again.
Instead of doing that, however, this CL just punts and
uses unoptimized functions everywhere on Plan 9.
LGTM=rsc
R=rsc, 0intro
CC=golang-codereviews
https://golang.org/cl/
73830044