runtime: drop SIGPROF while in ARM < 7 kernel helpers

On Linux ARMv6 and below runtime/internal/atomic.Cas calls into a kernel
cas helper at a fixed address. If a SIGPROF arrives while executing the
kernel helper, the sigprof lostAtomic logic will miss that we are
potentially in the spinlock critical section, which could cause
a deadlock when using atomics later in sigprof.

Fixes #47505

Change-Id: If8ba0d0fc47e45d4e6c68eca98fac4c6ed4e43c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/341889


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>