Introduce support for Debug Registers in RegisterContextNetBSD_x86_64

Summary:
NetBSD 7.99.62 introduced Debug Registers interface similar to the FreeBSD one.
This interface will land NetBSD-8.0.

Introduce support for this interface in Register Context NetBSD x86_64 unconditionally as older versions of NetBSD will not be supported.

This change allows to reduce diff with other ports and remove local copy of the RegisterInfos_x86_64.h content.

NetBSD Register Context for 32-bit x86 support will be added later.

Sponsored by <The NetBSD Foundation>

Reviewers: labath, joerg, emaste, clayborg

Reviewed By: labath, clayborg

Subscribers: #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D30287

llvm-svn: 296071