This patch fixes more cases of possible preempt issue when testing
MSR for FP or VEC bits and then doing giveup_fpu or giveup_altivec
that I missed in my previous round of fixes (bk get helps before
grepping ;)

I also change the single step and program check exceptions to not
re-enable interrupts right away on C code entry, it was useless and
would cause interesting issues with preempt & xmon