Use fcntl64Syscall in forkAndExecInChild1 to get fcntl64 on 32-bit Linux
systems.

Updates #36211

Change-Id: Id0e34359256beace970e72102fdace7a987ff2b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/212598
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>