runtime: correct semaphore implementation on netbsd

runtime: correct semaphore implementation on netbsd

NetBSD's semaphores use the underlying lighweight process mechanism
(LWP) on NetBSD, rather than pthreads. This means the m.prodcid needs
to be set to the LWP ID rather than the pthread ID in order for unpark
notifications to get sent to the right place.

Introduce a new getProcID() method that selects the correct ID for the
platform.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/261742