src: abstract getpid() operation

There are a few places where we paper over the fact that getpid() is
called GetCurrentProcessId() on Windows.  Let's move it into a function.

PR-URL: https://github.com/nodejs/node/pull/17087


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>