target-ppc: Don't invalidate non-supported msr bits

The msr invalidation code (commits 993eb and 2360b) inverts all
bits except MSR_TGPR and MSR_HVB. On non PowerPC 601 processors
this leads to incorrect change of excp_prefix in hreg_store_msr()
function. The problem is that new msr value get multiplied by msr_mask
and inverted msr does not, thus values of MSR_EP bit in new msr value
and inverted msr are distinct, so that excp_prefix changes but should
not.

Signed-off-by: Kurban Mallachiev <mallachiev@ispras.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
