Add basic OpenBSD support. This is enough to be able to analyze core dumps for OpenBSD/amd64, OpenBSD/arm, OpenBSD/arm64 and OpenBSD/i386.
Note that part of the changes to source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp fix a bug that probably affects other platforms as well. The GetProgramHeaderByIndex() interface use 1-based indices, but in some case when looping over the headers the, the loop starts at 0 and misses the last header. This caused problems on OpenBSD since OpenBSD core dumps have the PT_NOTE segment as the last program header.