target-xtensa: implement instruction breakpoints

Add IBREAKA/IBREAKENABLE SRs and implement debug exception, BREAK and
BREAK.N instructions and IBREAK breakpoints.

IBREAK breakpoint address is considered constant for TB lifetime.
On IBREAKA/IBREAKENABLE change corresponding TBs are invalidated.

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