[POWERPC] Fix bootwrapper builds with newer gcc versions

GCC 4.4.x looks to be adding support for generating out-of-line register
saves/restores based on:

http://gcc.gnu.org/ml/gcc-patches/2008-04/msg01678.html

This breaks the bootwrapper as we'd need to link with libgcc to get the
implementation of the register save/restores.

To workaround this issue, we just stole the save/restore code from gcc
and simplified it down for our needs (integer only).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>