Use generic implementation of IndexByte/IndexByteString
on plan9/amd64 since the assembly implementation
uses SSE instructions which are classified as floating
point instructions and cannot be used in a note handler.
A similar issue was fixed in CL 100577.

This fixes runtime.TestBreakpoint.

Fixes #61087.

Change-Id: Id0c085e47da449be405ea04ab9b93518c4e2fde8
Reviewed-on: https://go-review.googlesource.com/c/go/+/508400


Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>