Support building of system image in binary builds

This commit adds a few new pieces of functionality:

* The `contrib/build_sysimg.jl` script which builds a new system image.  This method can save the system image wherever the user desires, e.g. it could be stored in `~/.julia`, to allow for per-user system images each customized with packages in their own `userimg.jl`, for instance.  Or on a restricted system, this allows for creation of a system image without root access.

* The removal of compile-time `JULIA_CPU_TARGET`, in favor of runtime `--cpu-target`/`-C` command-line flags which default to `"native"` but can be set to `"native"`, `"i386"` or `"core2"`.  This allows the creation of a system image targeting user-supplied cpu features, e.g. `cd base; ../julia -C i386 --build /tmp/sys_i386 sysimg.jl`.

* I implemented runtime selection of the cpu target by adding a new member to the `jl_compileropts_t` structure called `cpu_target`.

* Because all julia executables are now created equal, (rather than before where a julia executable needed to have the same `JULIA_CPU_TARGET` set internally as the system image had when it was built) we need to know what CPU feature set the system image is targeting before we initialize code generation.  So a new function `jl_get_system_image_cpu_target()` is exported, which does exactly what it sounds like.

* I added newlines to the end of a few error messages.

* I found an old parser option `-T` which hadn't been removed yet, so I took the opportunity to do so.

When testing this change out, I found [this gist](https://gist.github.com/staticfloat/93d7050a08ff7bb52373) helpful to put into my `~/.juliarc.jl`
This warning probably shouldn't be conditional on llvm version any more. I'd love to get this script into 0.3.4, which we can ship without sys.dll because of the missing line numbers in the backtraces, but with this mechanism for users to regenerate it on demand.