AVR: target/112944 - Addendum: Link code to initialize NVMCTRL_CTRLB.FLMAP

For devices that see a part for the flash memory in the RAM address space,
bit-field NVMCTRL_CTRLB.FLMAP must match the value of symbol __flmap.
This is achieved by dragging in startup code from lib<mcu>.a.
The mechanism is the same like for libgcc's __do_copy_data and __do_clear_bss.
The code is implemented in AVR-LibC #931 and can be dragged by referencing
__do_flmap_init.

In addition to setting FLMAP, that code also sets bit FLMAPLOCK provided
symbol __flmap_lock has a non-zero value.  This protects FLMAP from future
changes.

When the __do_flmap_init code is not wanted, the symbol can be satisfied by
linking with  -Wl,--defsym,__do_flmap_init=0

gcc/
	PR target/112944
	* config/avr/gen-avr-mmcu-specs.cc (print_mcu) [have_flmap]:
	<*link_rodata_in_ram>: Spec undefs symbol __do_flmap_init
	when not linked with -mrodata-in-ram.