selftests/bpf: Fix cross compilation with CLANG_CROSS_FLAGS

I cross-compile my BPF selftests with the following command:

CLANG_CROSS_FLAGS="--target=aarch64-linux-gnu --sysroot=/sysroot/" \
  make LLVM=1 CC=clang CROSS_COMPILE=aarch64-linux-gnu- SRCARCH=arm64

(Note the use of CLANG_CROSS_FLAGS to specify a custom sysroot instead
of letting clang use gcc's default sysroot)

However, CLANG_CROSS_FLAGS gets propagated to host tools builds (libbpf
and bpftool) and because they reference it directly in their Makefiles,
they end up cross-compiling host objects which results in linking
errors.

This patch ensures that CLANG_CROSS_FLAGS is reset if CROSS_COMPILE
isn't set (for example when reaching a BPF host tool build).

Signed-off-by: Florent Revest <revest@chromium.org>
Link: https://lore.kernel.org/r/20230217151832.27784-1-revest@chromium.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/scripts/Makefile.include

@@ -108,6 +108,8 @@ endif # GCC_TOOLCHAIN_DIR
 endif # CLANG_CROSS_FLAGS
 CFLAGS += $(CLANG_CROSS_FLAGS)
 AFLAGS += $(CLANG_CROSS_FLAGS)
+else
+CLANG_CROSS_FLAGS :=
 endif # CROSS_COMPILE
 
 # Hack to avoid type-punned warnings on old systems such as RHEL5: