runtime: use bytes.IndexByte in findnull

runtime: use bytes.IndexByte in findnull

bytes.IndexByte is heavily optimized. Use it in findnull.
This is second attempt, similar to CL97523.
In this version we never call IndexByte on region of memory,
that crosses page boundary. A bit slower than CL97523,
but still fast:

name        old time/op  new time/op  delta
GoString-6   164ns ± 2%   118ns ± 0%  -28.00%  (p=0.000 n=10+6)

findnull is also used in gostringnocopy,
which is used in many hot spots in the runtime.

Fixes #23830

Change-Id: Id843dd4f65a34309d92bdd8df229e484d26b0cb2
Reviewed-on: https://go-review.googlesource.com/98015
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>