Update avrxmega3 linker emulation to support avrxmega2 devices with flash memory...

Update avrxmega3 linker emulation to support avrxmega2 devices with flash memory visible in the SRAM address range.

    PR ld/21472
ld  * emulparams/avrxmega3.sh (RODATA_PM_OFFSET): Set to 0x8000.
    * scripttempl/avr.sc
    (__RODATA_PM_OFFSET__) [RODATA_PM_OFFSET]: Use RODATA_PM_OFFSET
    as default if not already defined.
    (.data) [!RODATA_PM_OFFSET]: Don't include .rodata and friends.
    (.rodata) [RODATA_PM_OFFSET]: Put at an offset of
    __RODATA_PM_OFFSET__.

gas * config/tc-avr.c (mcu_types): Add entries for: attiny416,
     attiny417, attiny816, attiny817.