Improve architectural compliance of mfspr and mtspr

Improve architectural compliance of mfspr and mtspr

Mfspr from an unimplemented SPR should be a no-op in privileged state,
so in this case we need to write back whatever was previously in the
destination register.  For problem state, both mtspr and mfspr to
unimplemented SPRs should cause a program interrupt.

There are special cases in the architecture for SPRs 0, 4 5 and 6
which we still don't implement.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>