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.
AVR/ld: Propagate link-relax elf header flag correctly.

The AVR target has an elf header flag to indicate if an object was
assembler ready for linker relaxation.  If a partial link is performed
then it is important that the link-relax flag in the output object is
set correctly, otherwise, during the final link, we might try to perform
linker relaxation on code that was not assembled suitably.

As the link-relax elf header covers the entire object file we must be
conservative when setting the flag in the output object, so, for a
partial link, any input object that does not have the link-relax flag
set will cause the output object to also not have the link-relax flag
set.

This conservative approach could be softened in future, we only need to
disable the link relax flag if an input file is not marked link-relax
ready, and the input file contains a relaxable section.  However, I've
left this optimisation for a later day.

For the final link I've overloaded the use of the link-relax elf header
flag, in a final executable, the flag now indicates if the executable
was built with linker relaxation on or not.

ld/ChangeLog:

	* emultempl/avrelf.em: Add include of elf/avr.h.
	(avr_finish): New function.
	(LDEMUL_FINISH): Added.

ld/testsuite/ChangeLog:

	* ld-avr/relax-elf-flags-01.d: New file.
	* ld-avr/relax-elf-flags-02.d: New file.
	* ld-avr/relax-elf-flags-03.d: New file.
	* ld-avr/relax-elf-flags-04.d: New file.
	* ld-avr/relax-elf-flags-05.d: New file.
	* ld-avr/relax-elf-flags-06.d: New file.
	* ld-avr/relax-elf-flags-07.d: New file.
	* ld-avr/relax-elf-flags-08.d: New file.
	* ld-avr/relax-elf-flags-a.s: New file.
	* ld-avr/relax-elf-flags-b.s: New file.