CMake: Fuzz: perform link-time dead code elimination
CMake: Fuzz: perform link-time dead code elimination

Results in somewhat filesize-smaller fuzz targets,
and reduces the number of fuzz edge coverage counters,
since the unreachables are stripped away.

darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed