powerpc: Fix might-sleep warning in program check exception handler

On 32-bit, the exception prolog for the program check exception doesn't
enable interrupts early on.  If it is an illegal instruction exception,
we read the instruction in order to emulate certain instructions, and
the get_user of the instruction triggers a WARN_ON since interrupts
are still disabled.  This adds a local_irq_enable() to enable
interrupts before reading the instruction.

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