target/xtensa: add generic instruction post-processing

target/xtensa: add generic instruction post-processing

Some opcodes may need additional actions at every exit from the
translated instruction or may need to amend TB exit slots available to
jumps generated for the instruction. Add gen_postprocess function and
call it from the gen_jump_slot and from the disas_xtensa_insn.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>