Big-endian targets: Don't ignore offset into DW_OP_stack_value

Big-endian targets: Don't ignore offset into DW_OP_stack_value

Recently I fixed a bug that caused a DW_OP_implicit_pointer with non-zero
offset into a DW_OP_implicit_value to be handled incorrectly on big-endian
targets.  GDB ignored the offset and copied the wrong bytes:

  https://sourceware.org/ml/gdb-patches/2017-01/msg00251.html

But there is still a similar issue when a DW_OP_implicit_pointer points
into a DW_OP_stack_value instead; and again, the offset is ignored.  There
is an important difference, though: While implicit values are treated like
blocks of data and anchored at the lowest-addressed byte, stack values
traditionally contain integer numbers and are anchored at the *least
significant* byte.  Also, stack values do not come in varying sizes, but
are cut down appropriately when used.  Thus, on big-endian targets the
scenario looks like this (higher addresses shown right):

  |<- - - - - Stack value - - - - - - ->|
                  |                     |
                  |<- original object ->|
                  |
                  | offset ->|####|
			      ^^^^
                              de-referenced
			      implicit pointer

(Note how the original object's size influences the position of the
de-referenced implicit pointer within the stack value.  This is not the
case for little-endian targets, where the original object starts at offset
zero within the stack value.)

This patch implements the logic indicated in the above diagram and adds an
appropriate test case.  A new function dwarf2_fetch_die_type_sect_off is
added; it is used for retrieving the original object's type, so its size
can be determined.  That type is passed to dwarf2_evaluate_loc_desc_full
via a new parameter.

gdb/ChangeLog:

	* dwarf2loc.c (indirect_synthetic_pointer): Get data type of
	pointed-to DIE and pass it to dwarf2_evaluate_loc_desc_full.
	(dwarf2_evaluate_loc_desc_full): New parameter subobj_type; rename
	byte_offset to subobj_byte_offset.  Fix the handling of
	DWARF_VALUE_STACK on big-endian targets when coming via an
	implicit pointer.
	(dwarf2_evaluate_loc_desc): Adjust call to
	dwarf2_evaluate_loc_desc_full.
	* dwarf2loc.h (dwarf2_fetch_die_type_sect_off): New declaration.
	* dwarf2read.c (dwarf2_fetch_die_type_sect_off): New function.

gdb/testsuite/ChangeLog:

	* lib/dwarf.exp: Add support for DW_OP_implicit_pointer.
	* gdb.dwarf2/nonvar-access.exp: Add test for stack value location
	and implicit pointer into such a location.