selftests/bpf: Include build flavors for install target

When using the "install" or targets depending on install, e.g. "gen_tar",
the BPF machine flavors weren't included.

A command like:
  | make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- O=/workspace/kbuild \
  |    HOSTCC=gcc FORMAT= SKIP_TARGETS="arm64 ia64 powerpc sparc64 x86 sgx" \
  |    -C tools/testing/selftests gen_tar
would not include bpf/no_alu32, bpf/cpuv4, or bpf/bpf-gcc.

Include the BPF machine flavors for "install" make target.
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230831162954.111485-1-bjorn@kernel.org

tools/testing/selftests/bpf/Makefile

@@ -50,14 +50,17 @@ TEST_GEN_PROGS = test_verifier test_tag test_maps test_lru_map test_lpm_map test
 	test_cgroup_storage \
 	test_tcpnotify_user test_sysctl \
 	test_progs-no_alu32
+TEST_INST_SUBDIRS := no_alu32
 
 # Also test bpf-gcc, if present
 ifneq ($(BPF_GCC),)
 TEST_GEN_PROGS += test_progs-bpf_gcc
+TEST_INST_SUBDIRS += bpf_gcc
 endif
 
 ifneq ($(CLANG_CPUV4),)
 TEST_GEN_PROGS += test_progs-cpuv4
+TEST_INST_SUBDIRS += cpuv4
 endif
 
 TEST_GEN_FILES = test_lwt_ip_encap.bpf.o test_tc_edt.bpf.o
@@ -714,3 +717,12 @@ EXTRA_CLEAN := $(TEST_CUSTOM_PROGS) $(SCRATCH_DIR) $(HOST_SCRATCH_DIR)	\
 
 # Delete partially updated (corrupted) files on error
 .DELETE_ON_ERROR:
+
+DEFAULT_INSTALL_RULE := $(INSTALL_RULE)
+override define INSTALL_RULE
+	$(DEFAULT_INSTALL_RULE)
+	@for DIR in $(TEST_INST_SUBDIRS); do		  \
+		mkdir -p $(INSTALL_PATH)/$$DIR;   \
+		rsync -a $(OUTPUT)/$$DIR/*.bpf.o $(INSTALL_PATH)/$$DIR;\
+	done
+endef