#628 Deprecate enum Dwarf_ISA in favour EM_* values

[Joseph Koshy]

libdwarf/libdwarf.h defines an enum Dwarf_ISA that is only used by our extension API dwarf_producer_set_isa().

We could as well use the values of the EM_* constants to specify the ISA desired -- the use of an independently maintained enum doesn't seem to be adding value.