[IA64] rename _bss to __bss_start

Rename _bss to __bss_start as on other architectures.  That makes it
possible to use the <linux/sections.h> instead of own declarations.  Also
add __bss_stop because that symbol exists on other architectures.

Signed-off-by: Bernhard Walle <bwalle@suse.de>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>