xtensa: add support for SSP

xtensa: add support for SSP

gcc/
2017-05-08  Max Filippov  <jcmvbkbc@gmail.com>

	* config/xtensa/xtensa-protos.h
       	(xtensa_initial_elimination_offset): New declaration.
	* config/xtensa/xtensa.c (xtensa_initial_elimination_offset):
	New function. Move its body from the INITIAL_ELIMINATION_OFFSET
	macro definition, add case for FRAME_POINTER_REGNUM when
	FRAME_GROWS_DOWNWARD.
	* config/xtensa/xtensa.h (FRAME_GROWS_DOWNWARD): New macro
	definition.
	(INITIAL_ELIMINATION_OFFSET): Replace body with call to
	xtensa_initial_elimination_offset.

From-SVN: r247771