runtime/internal/atomic: remove double-check in kernelcas on linux/arm

runtime/internal/atomic: remove double-check in kernelcas on linux/arm

Older kernels which require the double check of the __kuser_cmpxchg
result are no longer supported as of Go 1.18 which requires at least
Linux 2.6.32.

For #45964

Change-Id: Ic3d6691bf006353ac51b9d43e742d970e3d4e961
Reviewed-on: https://go-review.googlesource.com/c/go/+/424556
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>