The SSA compiler currently generates MOVOstore instructions
to optimize 16 bytes moves on AMD64 architecture.

However, we can't use the MOVOstore instruction on Plan 9,
because floating point operations are not allowed in the
note handler.

We rely on the useSSE flag to disable the use of the
MOVOstore instruction on Plan 9 and replace it by two
MOVQstore instructions.

Fixes #21625

Change-Id: Idfefcceadccafe1752b059b5fe113ce566c0e71c
Reviewed-on: https://go-review.googlesource.com/59171
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>