[dev.typeparams] cmd/compile, runtime: do not zero X15 on Plan 9

On Plan 9, we cannot use SSE registers in note handlers, so we
don't use X15 for zeroing (MOVOstorezero and DUFFZERO). Do not
zero X15 on Plan 9.

Change-Id: I2b083b01b27965611cb83d19afd66b383dc77846
Reviewed-on: https://go-review.googlesource.com/c/go/+/321329


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>