Graceful degrade if Binutils PR21472 is not available.

Graceful degrade if Binutils PR21472 is not available.

gcc/
	Graceful degrade if Binutils PR21472 is not available.
	PR target/81072
	* configure.ac [target=avr]: WARN instead of ERROR if avrxmega3
	.rodata in flash test fails.
	(HAVE_LD_AVR_AVRXMEGA3_RODATA_IN_FLASH): Define it if test passes.
	* confgure: Regenerate.
	* config.in: Regenerate.
	* config/avr/avr.c (avr_asm_named_section)
	[HAVE_LD_AVR_AVRXMEGA3_RODATA_IN_FLASH]: Only trigger
	__do_copy_data for stuff in .rodata if flash_pm_offset = 0.
	(avr_asm_init_sections): Same.

From-SVN: r250000