x86_64: ia32entry adjustments

Consolidate the three 32-bit system call entry points so that they all
treat registers in similar ways.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>