arc: Add DWARF2 alternate CFA column.

arc: Add DWARF2 alternate CFA column.

Add DWARF 2 CFA column which tracks the return address from a signal
handler context.  This value must not correspond to a hard register
and must be out of the range of DWARF_FRAME_REGNUM().

gcc/
	* config/arc/arc.h (DWARF_FRAME_REGNUM): Update definition.
	(DWARF_FRAME_RETURN_COLUMN): Use RETURN_ADDR_REGNUM macro.
	(INCOMING_RETURN_ADDR_RTX): Likewise.
	(DWARF_ALT_FRAME_RETURN_COLUMN): Define.

gcc/testsuite/
	* gcc.target/arc/cancel-1.c: New file.

libgcc/
	* config/arc/linux-unwind.h (arc_fallback_frame_state): Use
	DWARF_ALT_FRAME_RETURN_COLUMN macro.

Signed-off-by: Claudiu Zissulescu <claziss@synopsys.com>