selftests/bpf: Enforce no-ALU32 for test_progs-no_alu32

With the most recent Clang, alu32 is enabled by default if -mcpu=probe or
-mcpu=v3 is specified. Use a separate build rule with -mcpu=v2 to enforce no
ALU32 mode.

Suggested-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20191120002510.4130605-1-andriin@fb.com

tools/testing/selftests/bpf/Makefile

@@ -163,6 +163,12 @@ define CLANG_BPF_BUILD_RULE
 		-c $1 -o - || echo "BPF obj compilation failed") | 	\
 	$(LLC) -march=bpf -mcpu=probe $4 -filetype=obj -o $2
 endef
+# Similar to CLANG_BPF_BUILD_RULE, but with disabled alu32
+define CLANG_NOALU32_BPF_BUILD_RULE
+	($(CLANG) $3 -O2 -target bpf -emit-llvm				\
+		-c $1 -o - || echo "BPF obj compilation failed") | 	\
+	$(LLC) -march=bpf -mcpu=v2 $4 -filetype=obj -o $2
+endef
 # Similar to CLANG_BPF_BUILD_RULE, but using native Clang and bpf LLC
 define CLANG_NATIVE_BPF_BUILD_RULE
 	($(CLANG) $3 -O2 -emit-llvm					\
@@ -275,6 +281,7 @@ TRUNNER_BPF_LDFLAGS := -mattr=+alu32
 $(eval $(call DEFINE_TEST_RUNNER,test_progs))
 
 # Define test_progs-no_alu32 test runner.
+TRUNNER_BPF_BUILD_RULE := CLANG_NOALU32_BPF_BUILD_RULE
 TRUNNER_BPF_LDFLAGS :=
 $(eval $(call DEFINE_TEST_RUNNER,test_progs,no_alu32))