add jl_get_cpu_name function to libjulia

thin ccall'able wrapper around llvm::sys::getHostCPUName
Modify jl_get_cpu_name to use jl_pchar_to_string,

add Sys.cpu_name
Move cpu_name into init_sysinfo()
Merge pull request #7552 from tkelman/getcpuname

RFC: add jl_get_cpu_name function to libjulia