unix/Makefile: Disable assertions in the standard unix executable.

Reasons to disable:
- the code is relatively robust so doesn't need full checking in the
  main executable, and the coverage build is used for full testing
  with assertions still enabled;
- reduces code size noticeably, by 27k for x86-64 and 20k for x86;
- allows to more easily track changes in code size, since assertions
  can skew things.