runtime: throw in unreachable exitThread

runtime: throw in unreachable exitThread

Several OSes don't ever reach exitThread, On AIX, Plan9, Solaris, and
Windows, we throw if this function is accidentally reached. Do the same
on Darwin and OpenBSD for consistency.

Change-Id: Icd189b11179755a28b3ec48b267349c57facbf24
Reviewed-on: https://go-review.googlesource.com/c/go/+/443717
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>