Handle PIE in .debug_loclists

Simon pointed out that my recent patches to .debug_loclists caused
some regressions.  After a brief discussion we realized it was because
his system compiler defaults to PIE.

This patch changes this code to unconditionally apply the text offset
here.  It also changes loclist_describe_location to work more like
dwarf2_find_location_expression.

I tested this by running the gdb.dwarf2 tests both with and without
-pie.