[POWERPC] Correct masks used in emulating some instructions

When we get an illegal instruction exception, we check to see whether
the instruction is one that we emulate for the user program.  Some of
the masks we use in checking whether the offending instruction is one
we care about didn't have the top bit set, which is the MSB of the
major opcode.  Thus some undefined opcodes could get emulated as other
(defined but unimplemented) instructions.  This corrects the masks.

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