PPC: Fix GDB read on code area for PPC6xx

On PPC 6xx, data and code have separated TLBs. Until now QEMU was only
looking at data TLBs, which is not good when GDB wants to read code.

This patch adds a second call to get_physical_address() with an
ACCESS_CODE type of access when the first call with ACCESS_INT fails.

Signed-off-by: Fabien Chouteau <chouteau@adacore.com>
Signed-off-by: Alexander Graf <agraf@suse.de>