xtensa: SMP: rework IPI processing

Don't skip current CPU in send_ipi_message: callers of this function
take care of it and it's harmless anyway.
Don't clear IPI bits one by one, clear all that were read at once.
Check IPI register in a loop in case new IPI was posted while previous
was being handled.

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