Revert "runtime: treat SI_TKILL like SI_USER on Linux"

Revert "runtime: treat SI_TKILL like SI_USER on Linux"

This reverts CL 431255.

Reason for revert: breaks darwin-arm and linux-noopt builders.

Change-Id: I29332b935cc1e35fa039af3d70465e496361fcc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/431715
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>