powerpc/booke64: Fix exception numbers

altivec_unavailable was commented as 0xf20 but the code uses 0x200.
Note that 0xf20 is also used by ap_unavailable.

altivec_assist was commented as 0x1700 but the code uses 0x220.

critical_input was commented as 0x580 but the code uses 0x100.

machine_check was commented and implemented as 0x200, which conflicts
with altivec_assist (it only builds because MC_EXCEPTION_PROLOG is
commented out).  Changed to the fixed IVOR value of 0x000.

Signed-off-by: Scott Wood <scottwood@freescale.com>