Add support for DW_OP_bit_piece and DW_OP_plus_uconst to DWARF assembler.

  https://sourceware.org/ml/gdb-patches/2013-12/msg00143.html

gdb/testsuite/ChangeLog

	* lib/dwarf.exp: (Dwarf::_location): Handle DW_OP_bit_piece and
	DW_OP_plus_uconst.