powerpc: Fix bug in move of altivec code to vector.S

The patch that moved to vector.S and made common between 32 and 64-bit the
altivec code had a nasty bug on 32-bit (did I really test that ?) which
causes the kernel to blr back into userspace ... oops :-)

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>