Recognize Real-Time Signals on NetBSD

Summary:
Real-Time Signals are 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: labath, joerg, clayborg, emaste

Reviewed By: labath, clayborg, emaste

Subscribers: #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D29091

llvm-svn: 293391