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.
Hrm, wrapping it in `bytestring` from Julia worked in my testing on Linux and Windows, but you're right if LLVM is underspecifying its behavior than it's probably not safe to rely on it. Suggestions? Create a new string that we can force to be null-terminated and copy over the rest of the contents?