[release-branch.go1.23] runtime/cgo: avoid errors from -Wdeclaration-after-statement

It's used by the SWIG CI build, at least, and it's an easy fix.

[The change in x_cgo_sys_thread_create is gone, as that function was
already compliant in 1.23.]

Fixes #71962
For #71961

Change-Id: Id21071a5aef216b35ecf0e9cd3e05d08972d92fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/652181


Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
(cherry picked from commit 76c70282)
Reviewed-on: https://go-review.googlesource.com/c/go/+/652935


Reviewed-by: Michael Knyszek <mknyszek@google.com>