The presence of such a register — eg. rcx, which is actually used in CFA
on a few lines of libgcc_s.so on my system — will yield an assert(0) in
the produced code, but will at least not fail to compile alltogether.
This is only representing a small fragment of Dwarf, **on purpose**: in
practice, the huge majority of Dwarf observed in the wild belongs to
this fragment.
