Correctly handle DW_LLE_start_end

When the code to handle DW_LLE_start_end was added (as part of some
DWARF 5 work), it was written to add the base address.  However, this
seems incorrect -- the DWARF standard describes this as an address,
not an offset from the base address.

This patch changes a couple of spots in dwarf2/loc.c to fix this
problem.  It then changes decode_debug_loc_addresses to return
DEBUG_LOC_OFFSET_PAIR instead, which preserves the previous semantics.

This only showed up on the RISC-V target internally, due to the
combination of DWARF 5 and a newer version of GCC.  I've updated a
couple of existing loclists test cases to demonstrate the bug.