powerpc: Fix module building for gcc 4.5 and 64 bit

Gcc 4.5 is now generating out of line register save and restore
in the function prefix and postfix when we use -Os.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>