x86: introduce a state stack for .arch

When using just slightly non-trivial combinations of .arch, it can be
quite useful to be able to go back to prior state without needing to
re-invoke perhaps many earlier directives and without needing to invoke
perhaps many "negative" ones. Like some other architectures allow
saving (pushing) and restoring (popping) present/prior state.

For now require the same .code<N> to be in effect for ".arch pop" that
was in effect for the corresponding ".arch push".

Also change the global "no_cond_jump_promotion" to be bool, to match the
new struct field.