runtime/cgo: create C pthreads in detached state

runtime/cgo: create C pthreads in detached state

Rather than explicitly calling pthread_detach.

Fixes #68850

Change-Id: I7b4042283f9feb5383bffd40fae6db6d23217f97
Reviewed-on: https://go-review.googlesource.com/c/go/+/605257
Commit-Queue: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>