add jl_get_cpu_name function to libjulia

thin ccall'able wrapper around llvm::sys::getHostCPUName

As @stevengj suggested here https://groups.google.com/forum/#!topic/julia-dev/EYXr9KkMWgI, this was rather trivial. I'm open to renaming or moving this anywhere more appropriate. Can also add the result of this into Julia's `Sys` module if people would like that.
Why not call `jl_pchar_to_string` and return a `jl_value_t*` instead of a `char*`?