syscall: clean up variable declarations in forkAndExecInChild

syscall: clean up variable declarations in forkAndExecInChild

The various forkAndExecInChild implementations have comments
explaining that they pre-declare variables to force allocations
to occur before forking, but then later use ":=" declarations
for additional variables.

To make it clearer that those ":=" declarations do not allocate,
we move their declarations up to the predeclared blocks.

For #57208.

Change-Id: Ie8cb577fa7180b51b64d6dc398169053fdf8ea97
Reviewed-on: https://go-review.googlesource.com/c/go/+/456516
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>