target/i386: do not include undefined bits in the AMD topoext leaf

target/i386: do not include undefined bits in the AMD topoext leaf

Commit d7c72735f61 ("target/i386: Add new EPYC CPU versions with updated
cache_info", 2023-05-08) ensured that AMD-defined CPU models did not
have the 'complex_indexing' bit set, but left it set in "-cpu host"
which uses the default ("legacy") cache information.

Reimplement that commit using a CPU feature, so that it can be applied
to all guests using a new machine type, independent of the CPU model.

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