xtensa: fix PR target/65416

xtensa: fix PR target/65416

The issue is caused by reordering of stack pointer update after stack
space allocation with instructions that write to the allocated stack
space. In windowed ABI register spill area for the previous call frame
is located just below the stack pointer and may be reloaded back into
the register file on movsp.
Implement allocate_stack pattern for windowed ABI configuration and
insert an instruction that prevents reordering of frame memory access
and stack pointer update.

gcc/
2018-06-19  Max Filippov  <jcmvbkbc@gmail.com>

	Backport from mainline
	2018-06-19  Max Filippov  <jcmvbkbc@gmail.com>

	* config/xtensa/xtensa.md (UNSPEC_FRAME_BLOCKAGE): New unspec
	constant.
	(allocate_stack, frame_blockage, *frame_blockage): New patterns.

From-SVN: r261764