selftests/bpf: propagate LLVM toolchain to runqslower build

The selftests/bpf invokes a nested make when building runqslower, but
LLVM toolchain version (clang/llvm-strip) is not propagated. As a
result, runqslower is built with system default clang, not respecting
specified LLVM version.

    # LLVM=-21 make -C tools/testing/selftests/bpf
    ...
    make feature_display=0 -C /bpf/tools/bpf/runqslower                        \
        OUTPUT=/bpf/tools/testing/selftests/bpf/tools/build/runqslower/        \
        BPFOBJ_OUTPUT=/bpf/tools/testing/selftests/bpf/tools/build/libbpf/     \
        BPFOBJ=/bpf/tools/testing/selftests/bpf/tools/build/libbpf/libbpf.a    \
        BPF_INCLUDE=/bpf/tools/testing/selftests/bpf/tools/include             \
        BPFTOOL_OUTPUT=/bpf/tools/testing/selftests/bpf/tools/build/bpftool/   \
        VMLINUX_BTF=/sys/kernel/btf/vmlinux BPF_TARGET_ENDIAN=--target=bpfel   \
        EXTRA_CFLAGS='-g -O0  ' EXTRA_LDFLAGS=' ' &&                           \
        cp  /bpf/tools/testing/selftests/bpf/tools/build/runqslower/runqslower \
            /bpf/tools/testing/selftests/bpf/runqslower
    clang -g -O2 --target=bpfel -I/bpf/tools/testing/selftests/bpf/tools/build/runqslower/ \
          -I/bpf/tools/testing/selftests/bpf/tools/include -I/bpf/tools/include/uapi       \
          -c runqslower.bpf.c -o /bpf/tools/testing/selftests/bpf/tools/build/runqslower/runqslower.bpf.o && \
          llvm-strip -g /bpf/tools/testing/selftests/bpf/tools/build/runqslower//runqslower.bpf.o
    /bin/sh: 1: clang: not found

Explicitly propagate CLANG and LLVM_STRIP to the runqslower sub-make so
that the LLVM toolchain selection from lib.mk is preserved.

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