syscall: use dup3 in forkAndExecInChild1 on all Linux platforms

The minimum required Linux kernel version for Go 1.18 will be changed to
2.6.32, see #45964. The dup3 syscall was added in 2.6.27, so the
fallback to use the dup2 syscall in forkAndExecInChild1 on some
platforms can be removed.

For #45964

Change-Id: I8e04d7b5b2488990a061a080ed64ea303ad048b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/347350


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>