syscall: on linux use accept4 in Accept, fall back to accept

Android seems to block the accept syscall in newer versions. Use accept4
instead on kernel versions that support it (Linux 2.6.28 and newer) and
fall back to accept on ENOSYS.

Fixes #45767

Change-Id: If190ace0e0213207fdaf6eeb79a5543ef18456de
Reviewed-on: https://go-review.googlesource.com/c/go/+/313769


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>