Use pseudosections for NT_FREEBSD_X86_SEGBASES core dump notes.

This includes adding pseudosections when reading a core dump as well
as support for writing out a core dump note from a pseudosection.

bfd/ChangeLog:

	* elf-bfd.h (elfcore_write_x86_segbases): New.
	* elf.c (elfcore_grok_freebsd_note): Add pseudosections for
	NT_FREEBSD_X86_SEGBASES register notes.
	(elfcore_write_x86_segbases): New.
	(elfcore_write_register_note): Write NT_FREEBSD_X86_SEGBASES
	register notes.