Add getauxval() compat for NetBSD

Summary:
getauxval() is not available on NetBSD and there is no a direct equivalent.

Add a function that implements the same semantics with NetBSD internals.

Reorder the GetPageSize() functions to prefer the sysctl approach for NetBSD.
It no longer makes a difference which approach is better. Avoid changing
conditional code path.

Reviewers: vitalybuka, dvyukov, mgorny, joerg

Reviewed By: vitalybuka

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D67329

llvm-svn: 371758