Add internal_sysctl() used by FreeBSD, NetBSD, OpenBSD and MacOSX

Summary:
Switch local sysctl(2) calls to internal_sysctl().

This is a preparation for introduction of interceptors for
the sysctl*() family of functions and switching `internal_sysctl*()`
to libc calls bypassing interceptors.

No functional change intended with this revision.

Reviewers: vitalybuka, joerg, kcc

Reviewed By: vitalybuka

Subscribers: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D51425

llvm-svn: 341181