Merge branch 'bpf: tools: support build selftests/bpf with clang'

Yonghong Song says:

====================

To build kernel with clang, people typically use
  make -j60 LLVM=1 LLVM_IAS=1
LLVM_IAS=1 is not required for non-LTO build but
is required for LTO build. In my environment,
I am always having LLVM_IAS=1 regardless of
whether LTO is enabled or not.

After kernel is build with clang, the following command
can be used to build selftests with clang:
  make -j60 -C tools/testing/selftests/bpf LLVM=1 LLVM_IAS=1

I am using latest bpf-next kernel code base and
latest clang built from source from
  https://github.com/llvm/llvm-project.git
Using earlier version of llvm may have compilation errors, see
  tools/testing/selftests/bpf
due to continuous development in llvm bpf features and selftests
to use these features.

To run bpf selftest properly, you need have certain necessary
kernel configs like at:
  bpf-next:tools/testing/selftests/bpf/config
(not that this is not a complete .config file and some other configs
 might still be needed.)

Currently, using the above command, some compilations
still use gcc and there are also compilation errors and warnings.
This patch set intends to fix these issues.
Patch #1 and #2 fixed the issue so clang/clang++ is
used instead of gcc/g++. Patch #3 fixed a compilation
failure. Patch #4 and #5 fixed various compiler warnings.

Changelog:
  v2 -> v3:
    . more test environment description in cover letter. (Sedat)
    . use a different fix, but similar to other use in selftests/bpf
      Makefile, to exclude header files from CXX compilation command
      line. (Andrii)
    . fix codes instead of adding -Wno-format-security. (Andrii)
  v1 -> v2:
    . add -Wno-unused-command-line-argument and -Wno-format-security
      for clang only as (1). gcc does not exhibit those
      warnings, and (2). -Wno-unused-command-line-argument is
      only supported by clang. (Sedat)
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>