Handle DW_OP_GNU_implicit_pointer in dwarf assembler

DW_OP_GNU_implicit_pointer refers to a DIE with an offset of different
sizes in different dwarf versions.  In v2, the size is the pointer size,
while in v3 and above, it is the ref_addr size.  This patch fixes
dwarf assembler to emit the correct size of offset.  We've already fixed
this size issue in gdb,
https://sourceware.org/ml/gdb-patches/2011-09/msg00451.html

gdb/testsuite:

2017-01-25  Yao Qi  <yao.qi@linaro.org>

	* lib/dwarf.exp (Dwarf::_location): Handle
	DW_OP_GNU_implicit_pointer with proper size.