runtime: do not use memmove in the Plan 9 signal handler
Fixes a regression introduced in revision
4cb93e2900d0.
That revision changed runtime·memmove to use SSE MOVOU
instructions for sizes between 17 and 256 bytes. We were
using memmove to save a copy of the note string during
the note handler. The Plan 9 kernel does not allow the
use of floating point in note handlers (which includes
MOVOU since it touches the XMM registers).
Arguably, runtime·memmove should not be using MOVOU when
GO386=387 but that wouldn't help us on amd64. It's very
important that we guard against any future changes so we
use a simple copy loop instead.
This change is extracted from CL
9796043 (since that CL
is still being ironed out).
R=rsc
CC=golang-dev
https://golang.org/cl/
34640045