mmu_helper.c is, for obvious reasons, almost entirely concerned with
softmmu builds of qemu.  However, it does contain one stub function which
is used when CONFIG_USER_ONLY=y - the user only versoin of
cpu_ppc_handle_mmu_fault, which always triggers an exception.  The entire
rest of the file is surrounded by #if !defined(CONFIG_USER_ONLY).

We clean this up by moving the user only stub into its own new file,
removing the ifdefs and building mmu_helper.c only when CONFIG_SOFTMMU
is set.  This also lets us remove the #define of cpu_handle_mmu_fault to
cpu_ppc_handle_mmu_fault - that name is only used from generic code for
user only - so we just name our split user version by the generic name.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>