ppc: Rewrite ppc_get_compat_smt_threads()

To continue consolidation of compatibility mode information, this rewrites
the ppc_get_compat_smt_threads() function using the table of compatiblity
modes in target-ppc/compat.c.

It's not a direct replacement, the new ppc_compat_max_threads() function
has simpler semantics - it just returns the number of threads the cpu
model has, taking into account any compatiblity mode it is in.

This no longer takes into account kvmppc_smt_threads() as the previous
version did.  That check wasn't useful because we check in
ppc_cpu_realizefn() that CPUs aren't instantiated with more threads
than kvm allows (or if we didn't things will already be broken and
this won't make it any worse).

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