selftests/bpf: Explicitly pass RESOLVE_BTFIDS to sub-make

When cross-compiling selftests/bpf, the resolve_btfids binary end up
in a different directory, than the regular resolve_btfids
builds. Populate RESOLVE_BTFIDS for sub-make, so it can find the
binary.
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20221115182051.582962-1-bjorn@kernel.org

tools/testing/selftests/bpf/Makefile

@@ -201,7 +201,7 @@ $(OUTPUT)/sign-file: ../../../../scripts/sign-file.c
 $(OUTPUT)/bpf_testmod.ko: $(VMLINUX_BTF) $(wildcard bpf_testmod/Makefile bpf_testmod/*.[ch])
 	$(call msg,MOD,,$@)
 	$(Q)$(RM) bpf_testmod/bpf_testmod.ko # force re-compilation
-	$(Q)$(MAKE) $(submake_extras) -C bpf_testmod
+	$(Q)$(MAKE) $(submake_extras) RESOLVE_BTFIDS=$(RESOLVE_BTFIDS) -C bpf_testmod
 	$(Q)cp bpf_testmod/bpf_testmod.ko $@
 
 DEFAULT_BPFTOOL := $(HOST_SCRATCH_DIR)/sbin/bpftool