Implement __kmp_is_address_mapped() for NetBSD

Summary:
Use the sysctl(3) function to check whether an address is mapped
into the address space.

Reviewers: mgorny, joerg, #openmp

Reviewed By: mgorny

Subscribers: openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D55549

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@348874 91177308-0d34-0410-b5e6-96231b3b80d8