elf-core: Parse vendor-specific notes

ELF notes contain a 'name' field, which specifies a vendor who defines
the format of the note.  Examples are 'FreeBSD' or 'GNU', or it may be
empty for generic notes.

Add a case for FreeBSD-specific notes, leaving Linux and GNU notes,
other vendor-specific notes, and generic notes to be handled by the
existing code for now.

Thanks to Samuel Jacob for reviewing and suggesting improvements.

llvm-svn: 186973