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.
According to the LLVM [StringRef](http://llvm.org/docs/doxygen/html/classllvm_1_1StringRef.html) documentation, the data "need not be null terminated".  If HostCPUName is not null-terminated, that would be a problem here because you don't return the length.