libdw, readelf: Don't handle DW_FORM_data16 as expression block/location.

libdw, readelf: Don't handle DW_FORM_data16 as expression block/location.

Also found by afl-fuzz on the varlocs testcase.
DW_FORM_data16 is constant form according to the DWARF5 spec.
But since it is 128bits it isn't really representable as Dwarf_Word.
So we treat it as block form. But we cannot treat it as an expression
block. Make sure readelf prints it as a regular block and that
dwarf_getlocation[s|_addr] doesn't treat it as location expression.

Signed-off-by: Mark Wielaard <mark@klomp.org>