Recognize FreeBSD core dump note for x86 segment base registers.

This core dump note contains the value of the base address of the %fs
and %gs segments for both i386 and amd64 core dumps.  It is primarily
useful in resolving the address of TLS variables in core dumps.

binutils/ChangeLog:

	* readelf.c (get_freebsd_elfcore_note_type): Handle
	NT_FREEBSD_X86_SEGBASES.

include/ChangeLog:

	* elf/common.h (NT_FREEBSD_X86_SEGBASES): Define.