The new m16.int64 file contains a new "negate8" macro (for 64-bit negation), as well as an improved version of "ph8". These have been added to the individual *.macros files, but if those are regenerated, m16.int64 should be used as an input to macgen ahead of m16.ORCA (which contains the original version of ph8).
This is a fairly straightforward adaptation of the strtol/strtoul code. The multiplication routine is modified to return the next 16 bits in X so that strtoull can detect overflows.