internal/poll,syscall: drop the fallback for accept4 on linux/arm

internal/poll,syscall: drop the fallback for accept4 on linux/arm

For #67001

Change-Id: Ic0b91cd393046c53e41908e8dbbe2ab5fb6b8a74
Reviewed-on: https://go-review.googlesource.com/c/go/+/588216
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>