selftests/bpf: propagate LLVM toolchain into runqslower sub-make

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]>

Author: HoyeonRhee

tools/testing/selftests/bpf/Makefile

@@ -306,6 +306,7 @@ endif
 
 $(OUTPUT)/runqslower: $(BPFOBJ) | $(DEFAULT_BPFTOOL) $(RUNQSLOWER_OUTPUT)
 	$(Q)$(MAKE) $(submake_extras) -C $(TOOLSDIR)/bpf/runqslower	       \
+		    CLANG=$(CLANG) LLVM_STRIP=$(LLVM_STRIP)		       \
 		    OUTPUT=$(RUNQSLOWER_OUTPUT) VMLINUX_BTF=$(VMLINUX_BTF)     \
 		    BPFTOOL_OUTPUT=$(HOST_BUILD_DIR)/bpftool/		       \
 		    BPFOBJ_OUTPUT=$(BUILD_DIR)/libbpf/			       \

tools/testing/selftests/lib.mk

@@ -8,6 +8,7 @@ LLVM_SUFFIX := $(LLVM)
 endif
 
 CLANG := $(LLVM_PREFIX)clang$(LLVM_SUFFIX)
+LLVM_STRIP := $(LLVM_PREFIX)llvm-strip$(LLVM_SUFFIX)
 
 CLANG_TARGET_FLAGS_arm          := arm-linux-gnueabi
 CLANG_TARGET_FLAGS_arm64        := aarch64-linux-gnu