ppc: Rewrite ppc_set_compat()

This rewrites the ppc_set_compat() function so that instead of open coding
the various compatibility modes, it reads the relevant data from a table.
This is a first step in consolidating the information on compatibility
modes scattered across the code into a single place.

It also makes one change to the logic.  The old code masked the bits
to be set in the PCR (Processor Compatibility Register) by which bits
are valid on the host CPU.  This made no sense, since it was done
regardless of whether our guest CPU was the same as the host CPU or
not.  Furthermore, the actual PCR bits are only relevant for TCG[1] -
KVM instead uses the compatibility mode we tell it in
kvmppc_set_compat().  When using TCG host cpu information usually
isn't even present.

While we're at it, we put the new implementation in a new file to make the
enormous translate_init.c a little smaller.

[1] Actually it doesn't even do anything in TCG, but it will if / when we
    get to implementing compatibility mode logic at that level.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
