In CL 98015, findnull was rewritten so it uses bytes.IndexByte.

This broke the build on plan9/amd64 because the implementation
of bytes.IndexByte on AMD64 relies on SSE instructions while
floating point instructions are not allowed in the note handler.

This change fixes findnull by using the former implementation
on Plan 9, so it doesn't use bytes.IndexByte.

Fixes #24387.

Change-Id: I084d1a44d38d9f77a6c1ad492773f0a98226be16
Reviewed-on: https://go-review.googlesource.com/100577
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>