The runqslower build invokes a nested make, but the selected LLVM
toolchain (via LLVM=-<version>) is not propagated. This causes the
sub-make to call the system-default 'clang' and 'llvm-strip' even when
a specific LLVM version is intended.

    # LLVM=-20 V=1 make -C tools/testing/selftests/bpf
    ...
    make -C tools/bpf/runqslower ...
    clang -g -O2 --target=bpfel -I... -c runqslower.bpf.c -o runqslower.bpf.o && \
          llvm-strip -g runqslower.bpf.o
    /bin/sh: 1: clang: not found

    (expected: clang-20 and llvm-strip-20)

Propagate CLANG and LLVM_STRIP to the sub-make to ensure LLVM version
consistency across all builds.

Signed-off-by: Hoyeon Lee <[email protected]>