Switch HostInfoNetBSD::GetProgramFileSpec to sysctl(7)

Summary:
Remove dependency on the proc (/proc) filesystem, which is optional.

KERN_PROC_PATHNAME is available in NetBSD-current and will land NetBSD 8.0.
Older stable versions of NetBSD will not be supported.

Sponsored by <The NetBSD Foundation>

Reviewers: emaste, joerg, labath, clayborg

Reviewed By: clayborg

Subscribers: #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D29089

llvm-svn: 293392