AVR: Tidy up gen-avr-mmcu-specs.cc

Some information was (re-)computed in different places.
This patch computes them in new struct McuInfo and passes
it around in order to provide the information.

gcc/
	* config/avr/gen-avr-mmcu-specs.cc (struct McuInfo): New.
	(main, print_mcu, diagnose_mrodata_in_ram): Pass it down.