target/i386: extend pg_mode to more CR0 and CR4 bits

In order to unify the two stages of page table lookup, we need
mmu_translate to use either the host CR0/EFER/CR4 or the guest's.
To do so, make mmu_translate use the same pg_mode constants that
were used for the NPT lookup.

This also prepares for adding 5-level NPT support, which however does
not work yet.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>