PR gdb/21226: Take DWARF stack value pieces from LSB end

PR gdb/21226: Take DWARF stack value pieces from LSB end

When taking a DW_OP_piece or DW_OP_bit_piece from a DW_OP_stack_value, the
existing logic always takes the piece from the lowest-addressed end, which
is wrong on big-endian targets.  The DWARF standard states that the
"DW_OP_bit_piece operation describes a sequence of bits using the least
significant bits of that value", and this also matches the current logic
in GCC.  For instance, the GCC guality test case pr54970.c fails on s390x
because of this.

This fix adjusts the piece accordingly on big-endian targets.  It is
assumed that:

* DW_OP_piece shall take the piece from the LSB end as well;

* pieces reaching outside the stack value bits are considered undefined,
  and a zero value can be used instead.

gdb/ChangeLog:

	PR gdb/21226
	* dwarf2loc.c (read_pieced_value): Anchor stack value pieces at
	the LSB end, independent of endianness.

gdb/testsuite/ChangeLog:

	PR gdb/21226
	* gdb.dwarf2/nonvar-access.exp: Add checks for verifying that
	stack value pieces are taken from the LSB end.