selftests/bpf: Disable warnings on unused flags for Clang builds

There exist compiler flags supported by GCC but not supported by Clang
(e.g. -specs=...). Currently, these cannot be passed to BPF selftests
builds, even when building with GCC, as some binaries (urandom_read and
liburandom_read.so) are always built with Clang and the unsupported
flags make the compilation fail (as -Werror is turned on).

Add -Wno-unused-command-line-argument to these rules to suppress such
errors.

This allows to do things like:

    $ CFLAGS="-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1" \
      make -C tools/testing/selftests/bpf

Without this patch, the compilation would fail with:

    [...]
    clang: error: argument unused during compilation: '-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1' [-Werror,-Wunused-command-line-argument]
    make: *** [Makefile:273: /bpf-next/tools/testing/selftests/bpf/liburandom_read.so] Error 1
    [...]

Signed-off-by: Viktor Malik <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Link: https://lore.kernel.org/bpf/2d349e9d5eb0a79dd9ff94b496769d64e6ff7654.1730449390.git.vmalik@redhat.com

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