xtensa.c (machine_function): Add vararg_a7_copy.

	* config/xtensa/xtensa.c (machine_function): Add vararg_a7_copy.
	(xtensa_copy_incoming_a7): Use start_sequence instead of
	push_to_sequence.  Stash insns in vararg_a7_copy for builtin_saveregs.
	(xtensa_builtin_saveregs): Place code from vararg_a7_copy at the start
	of the saveregs sequence.

From-SVN: r128443