add llvm-ar(1) to the build with its two dependencies libLLVMDlltoolDriver
and libLLVMLibDriver;

switch LLD_ARCHs to llvm-ar(1) by skipping the installation of binutils' ar(1)
and linking llvm-ar(1) to ar(1)

tested on amd64, i386, arm64 and mips64

ok patrick@, kettenis@