Make jl_cpu_string a runtime compiler option

* Move `jl_cpu_string` to `jl_compileropts.cpu_target`
* Add `--cpu-target`/`-C` command-line options to julia for forcing CPU instruction compatibility
* Update makefiles and `interactiveutils.jl` to pass `-C` when (re)building system image
* Temporarily disable checks against mismatching julia executable and system image
* Remove old unused '-T` option in `parse_opts()`