[lldb] [test/Register] Initial tests for regsets in core dumps

Add initial tests for reading register sets from core dumps.  This
includes a C++ program to write registers and dump core, resulting core
dumps for Linux, FreeBSD and NetBSD, and the tests to verify them.

The tests are split into generic part, verifying user-specified register
values, and coredump-specific tests that verify memory addresses that
differ for every dump.

At this moment, all platforms support GPRs and FPRs up to XMM for amd64
target.  The i386 target does not work on NetBSD at all, and is missing
FPRs entirely on FreeBSD.

Differential Revision: https://reviews.llvm.org/D91963