use_dll: add the “restrict” C99 keyword to all pointers to double in computation functions

This greatly facilitates the job of the compiler during the optimization pass,
since we promise that the various pointers do not overlap each other. It may
now be possible to reenable some of the optimization flags that were disabled
without sacrificing compilation time, but this needs more investigation.

For the gory details, see:
https://en.cppreference.com/w/c/language/restrict
https://cellperformance.beyond3d.com/articles/2006/05/demystifying-the-restrict-keyword.html