AVR: Add support for a Compact Vector Table (-mcvt).

Some AVR devices support a CVT:

-  Devices from the 0-series, 1-series, 2-series.
-  AVR16, AVR32, AVR64, AVR128 devices.

The support is provided by means of a startup code file
crt<mcu>-cvt.o from AVR-LibC v2.3 that can be linked instead
of the traditional crt<mcu>.o.

This patch adds a new command line option -mcvt that links
that CVT startup code (or issues an error when the device
doesn't support a CVT).

	PR target/118764
gcc/
	* config/avr/avr.opt (-mcvt): New target option.
	* config/avr/avr-arch.h (AVR_CVT): New enum value.
	* config/avr/avr-mcus.def: Add AVR_CVT flag for devices that
	support it.
	* config/avr/avr.cc (avr_handle_isr_attribute) [TARGET_CVT]: Issue
	an error when a vector number larger that 3 is used.
	* config/avr/gen-avr-mmcu-specs.cc (McuInfo.have_cvt): New property.
	(print_mcu) <*avrlibc_startfile>: Use crt<mcu>-cvt.o depending
	on -mcvt (or issue an error when the device doesn't support a CVT).
	* doc/invoke.texi (AVR Options): Document -mcvt.