VTK-m builds with separate function sections to allow smaller binaries

Consumers of VTK-m when enabling of dropping of unused functions will see VTK-m functions dropped. Previously this didn't happen as VTK-m didn't build object files with the correct flags for this.

By allowing the linker to remove unused symbols we see a significant saving the file size of VTK-m tests, examples, and benchmarks. An OpenMP build of the tests and benchmarks goes from 168MB to 141MB which is roughly a 16% filesize reduction.

Initially I had presumed that these changes would increase link times. But in measurements the total wall time for compilation of VTK-m has stayed about the same ( seeing a decrease of 1.5% ). Presumably the increased computation is offset by the reduction in file writing.