These devices see a 32 KiB block of their program memory (flash) in
the RAM address space.  This can be used to support .rodata in flash
provided Binutils support PR31124 (Add new emulations which locate
.rodata in flash).  This patch does the following:

* configure checks availability of Binutils PR31124.

* Add new command line options -mrodata-in-ram and -mflmap.
While -flmap is for internal usage (communicate hardware properties
from device-specs to the compiler proper), -mrodata-in-ram is a user
space option that allows to return to the current rodata-in-ram layout.

* Adjust gen-avr-mmcu-specs.cc so that device-specs are generated
that sanity check options, and that translate -m[no-]rodata-in-ram
to its emulation.

* Objects in .rodata don't drag __do_copy_data.

* Document new options and built-in macros.

	PR target/112944

gcc/
	* configure.ac [target=avr]: Check availability of emulations
	avrxmega2_flmap and avrxmega4_flmap, resulting in new config vars
	HAVE_LD_AVR_AVRXMEGA2_FLMAP and HAVE_LD_AVR_AVRXMEGA4_FLMAP.
	* configure: Regenerate.
	* config.in: Regenerate.
	* doc/invoke.texi (AVR Options): Document -mflmap, -mrodata-in-ram,
	__AVR_HAVE_FLMAP__, __AVR_RODATA_IN_RAM__.

	* config/avr/avr.opt (-mflmap, -mrodata-in-ram): New options.
	* config/avr/avr-arch.h (enum avr_device_specific_features):
	Add AVR_ISA_FLMAP.
	* config/avr/avr-mcus.def (AVR_MCU) [avr64*, avr128*]: Set isa flag
	AVR_ISA_FLMAP.
	* config/avr/avr.cc (avr_arch_index, avr_has_rodata_p): New vars.
	(avr_set_core_architecture): Set avr_arch_index.
	(have_avrxmega2_flmap, have_avrxmega4_flmap)
	(have_avrxmega3_rodata_in_flash): Set new static const bool according
	to configure results.
	(avr_rodata_in_flash_p): New function using them.
	(avr_asm_init_sections): Let readonly_data_section->unnamed.callback
	track avr_need_copy_data_p only if not avr_rodata_in_flash_p().
	(avr_asm_named_section): Track avr_has_rodata_p.
	(avr_file_end): Emit __do_copy_data also when avr_has_rodata_p
	and not avr_rodata_in_flash_p ().
	* config/avr/specs.h (CC1_SPEC): Add %(cc1_rodata_in_ram).
	(LINK_SPEC): Add %(link_rodata_in_ram).
	(LINK_ARCH_SPEC): Remove.
	* config/avr/gen-avr-mmcu-specs.cc (have_avrxmega3_rodata_in_flash)
	(have_avrxmega2_flmap, have_avrxmega4_flmap): Set new static
	const bool according to configure results.
	(diagnose_mrodata_in_ram): New function.
	(print_mcu): Generate specs with the following changes:
	<*cc1_misc, *asm_misc, *link_misc>: New specs so that we don't
	need to extend avr/specs.h each time we add a new bell or whistle.
	<*cc1_rodata_in_ram, *link_rodata_in_ram>: New specs to diagnose
	-m[no-]rodata-in-ram.
	<*cpp_rodata_in_ram>: New. Does -D__AVR_RODATA_IN_RAM__=0/1.
	<*cpp_mcu>: Add -D__AVR_AVR_FLMAP__ if it applies.
	<*cpp>: Add %(cpp_rodata_in_ram).
	<*link_arch>: Use emulation avrxmega2_flmap, avrxmega4_flmap as
	requested.
	<*self_spec>: Add -mflmap or %<mflmap as needed.

gcc/testsuite/
	* gcc.target/avr/torture/pr112944-flmap-0.c: New test.
	* gcc.target/avr/torture/pr112944-flmap-1.c: New test.