x86/ia32: fix build with DEBUG=1

GCC isn't happy if we use %rbp as register for local variable
with -ggdb3 option.
Which resulted in the following build error for `make DEBUG=1`:
> In file included from criu/arch/x86/crtools.c:10:0:
> criu/arch/x86/include/asm/compat.h: In function ‘do_full_int80’:
> criu/arch/x86/include/asm/compat.h:50:1: error: bp cannot be used in asm here

Fix it by saving/restoring %rbp around 32-bit syscall manually.
Just while at it - add a comment about r8-r11 clobbers.

Reported-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>