makesyscalls: permit a range of syscall numbers for UNIMPL

Some ABIs have large gaps in syscall numbers. Allow gaps to be filled
as ranges of UNIMPL, with an entry like:

248-1023 AUE_NULL UNIMPL unimplemented

Reviewed by: jhb, gnn
Sponsored by: Turing Robotic Industries Inc.
Differential Revision: https://reviews.freebsd.org/D14122