AVR: Only set link-relax elf flag when appropriate.

AVR: Only set link-relax elf flag when appropriate.

The AVR target uses a bit in the elf header flags to indicate if the
object was assembled ready for linker relaxation.  Previously this flag
was always set, even when the object was not assembled ready for linker
relaxation.

This patch moves setting of the flag into the assembler, and sets it
only when the assembler is preparing the file for linker relaxation.

bfd/ChangeLog:

	* elf32-avr.c (bfd_elf_avr_final_write_processing): Don't set
	EF_AVR_LINKRELAX_PREPARED unconditionally.

gas/ChangeLog:

	* config/tc-avr.c: Add include for elf/avr.h.
	(avr_elf_final_processing): New function.
	* config/tc-avr.h (elf_tc_final_processing): Define.
	(avr_elf_final_processing): Declare

gas/testsuite/ChangeLog:

	* gas/avr/link-relax-elf-flag-clear.d: New file.
	* gas/avr/link-relax-elf-flag-set.d: New file.
	* gas/avr/link-relax-elf-flag.s: New file.