Respect piece offset for DW_OP_bit_piece

Respect piece offset for DW_OP_bit_piece

So far GDB ignores the piece offset of all kinds of DWARF bit
pieces (DW_OP_bit_piece) and treats such pieces as if the offset was zero.

This is fixed, and an appropriate test is added.

gdb/ChangeLog:

	* dwarf2loc.c (read_pieced_value): Respect the piece offset, as
	given by DW_OP_bit_piece.
	(write_pieced_value): Likewise.

  Andreas Arnez  <arnez@linux.vnet.ibm.com>

	* gdb.dwarf2/var-access.exp: Add test for composite location with
	nonzero piece offsets.