re PR target/43746 (-fmerge-constants and -fmerge-all-constants don't work at AVR target)

gcc/
	PR target/43746
	* config/avr/elf.h (TARGET_ASM_SELECT_SECTION): Remove,
	i.e. use default_elf_select_section.
	(TARGET_HAVE_SWITCHABLE_BSS_SECTIONS): Remove.
	(READONLY_DATA_SECTION_ASM_OP): Remove.
	(TARGET_ASM_NAMED_SECTION): Move from here...
	* config/avr/avr.c: ...to here.
	(avr_asm_init_sections): Set unnamed callback of
	readonly_data_section.
	(avr_asm_named_section): Make static.
	
testsuite/
	PR target/43746
	* gcc.dg/array-quals-1.c: Don't xfail on AVR.

From-SVN: r176262