x86: move / quiesce pre-386 non-16-bit warning

x86: move / quiesce pre-386 non-16-bit warning

Emitting this warning for every insn, including ones having actual
errors, is annoying. Introduce a boolean variable to emit the warning
just once on the first insn after .arch may have changed the things, and
move the warning to output_insn(). (I didn't want to go as far as
checking whether the .arch actually turned off the i386 bit, but doing
so would be an option.)