Support AT_BSDFLAGS on FreeBSD.

FreeBSD's kernel recently added a new ELF auxiliary vector entry
holding a mask of software features provided by the kernel.  This
change fixes 'info auxv' to report the name and description for this
vector entry instead of '???'.

include/ChangeLog:

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

gdb/ChangeLog:

	* fbsd-tdep.c (fbsd_print_auxv_entry): Handle AT_FREEBSD_BSDFLAGS.