[runtime] Use getloadavg() on NetBSD as well

Switch NetBSD from reading /proc (which is broken) to getloadavg()
(which is already used by Darwin).  NetBSD discourages using procfs
in favor of system API calls.

Differential Revision: https://reviews.llvm.org/D55486

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@348885 91177308-0d34-0410-b5e6-96231b3b80d8