[patch,avr] Assert minimal required bit width of section_common::flags.

[Georg-Johann Lay]

This patch implements an alarm should we ever run out of
section flags bits.

Ok for trunk?

Johann

--

AVR: Assert minimal required bit width of section_common::flags.

gcc/
        * config/avr/avr.cc (avr_ctz): New constexpr function.
        (section_common::flags): Assert minimal bit width.