target/xtensa: support icount

target/xtensa: support icount

Delimit each instruction that may access timers or IRQ state with
qemu_io_start/qemu_io_end, so that qemu-system-xtensa could be run with
-icount option.

Raise EXCP_YIELD after CCOMPARE reprogramming to let tcg_cpu_exec
recalculate how long this CPU is allowed to run.

RSR now may need to terminate TB, but it can't be done in RSR handler
because the same handler is used for XSR together with WSR handler, which
may also need to terminate TB. Change RSR and WSR handlers return type
to bool indicating whether TB termination is needed (RSR) or has been
done (WSR), and add TB termination after RSR/WSR dispatcher call.

Signed-off-by: Max Filippov <[email protected]>