Enabled elf file loading on a platform other than the elf file's source.

ObjectFileElf assumed the vendor/os would always be the host vendor/os.
This is incorrect when, say, trying to debug Linux x86_64 llgs from
a MacOSX or FreeBSD lldb client.

ObjectFileElf now does some note parsing to refine its idea of the
object file architecture.  I implemented checks for Linux, FreeBSD
and NetBSD note sections.  I need to go back and use constants in
place of some numbers with comments.

This change also stops llgs from sending MachO cputype and cpusubtype when
llgs is not on an Apple host.