powerpc: Fix MSR setting in 32 bit signal code

If we set the SPE MSR bit in save_user_regs we can blow away the VEC
bit.  This doesn't matter in reality as they are in fact the same bit
but looks bad.

Also, when we add VSX in a later patch, we need to be able to set two
separate MSR bits here.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>