In the current samples/bpf/README.rst, the url of llvm and clang git
may be out of date, they are unable to access:
$ git clone http://llvm.org/git/llvm.git
Cloning into 'llvm'...
fatal: unable to access 'http://llvm.org/git/llvm.git/ ': Maximum (20) redirects followed
$ git clone --depth 1 http://llvm.org/git/clang.git
Cloning into 'clang'...
fatal: unable to access 'http://llvm.org/git/clang.git/ ': Maximum (20) redirects followed
There are different ways to build llvm/clang, I find the Clang Getting
Started page  has one way, as Yonghong said, it is better to just
copy the build procedure in Documentation/bpf/bpf_devel_QA.rst to keep
I verified the procedure and it is proved to be feasible, so we should
update README.rst to reflect the reality. At the same time, update the
related comment in Makefile.
Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>