[llvm] [Support] Reimplement getMainExecutable() using sysctl on NetBSD

Use sysctl() to implement getMainExecutable() on NetBSD, rather than
trying to guess the correct path from argv[0].  This is one
of the fixes to recent clang-check-mac-libcxx-fixed-compilation-db.cpp
test failure on NetBSD.

This has been historically done on both FreeBSD and NetBSD in r303015,
and reverted in r303285 due to buggy implementation on FreeBSD.
However, FWIK the NetBSD implementation does not suffer from the same
bugs and is more reliable than playing with argv[0].

Differential Revision: https://reviews.llvm.org/D56975

llvm-svn: 355283