Battery of NetBSD support improvements

Summary:
Include initial support for:
 - single step mode (PT_STEP)
 - single step trap handling (TRAP_TRACE)
 - exec() trap (TRAP_EXEC)
 - add placeholder interfaces for FPR
 - initial code for NetBSD core(5) files
 - minor tweaks

While there improve style of altered elf-core/ files.

This code raises the number of passing tests on NetBSD to around 50% (600+/1200+).

The introduced code is subject to improve afterwards for additional features and bug fixes.

Sponsored by <The NetBSD Foundation>

Reviewers: labath, joerg, emaste, kettenis

Reviewed By: labath

Subscribers: srhines, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D31450

llvm-svn: 299109