dwarf: Multi-register CFI address support

dwarf: Multi-register CFI address support

Add support for architectures such as AMD GCN, in which the pointer size is
larger than the register size.  This allows the CFI information to include
multi-register locations for the stack pointer, frame pointer, and return
address.

Note that this uses a newly proposed DWARF operator DW_OP_LLVM_piece_end,
which is currently only recognized by the ROCGDB debugger from AMD.  The exact
name and encoding for this operator is subject to change if and when the DWARF
standard accepts it.

gcc/ChangeLog:

	* dwarf2cfi.cc (get_cfa_from_loc_descr): Support register spans
	with DW_OP_piece and DW_OP_LLVM_piece_end.
	* dwarf2out.cc (build_cfa_loc): Support register spans.

include/ChangeLog:

	* dwarf2.def (DW_OP_LLVM_piece_end): New extension operator.