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

llvm-svn: 348874