The old code assumed that the thread ID set by pthread_create would be
available in the newly created thread.  While that is clearly true
eventually, it is not necessarily true immediately.  Rather than try to
pass down the thread ID, just call pthread_self in the created thread.

Fixes #15576 (I hope).

Change-Id: Ic07086b00e4fd5676c04719a299c583320da64a1
Reviewed-on: https://go-review.googlesource.com/22880
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>