selftests/bpf: make multi-uprobe tests work in RELEASE=1 mode

When BPF selftests are built in RELEASE=1 mode with -O2 optimization
level, uprobe_multi binary, called from multi-uprobe tests is optimized
to the point that all the thousands of target uprobe_multi_func_XXX
functions are eliminated, breaking tests.

So ensure they are preserved by using weak attribute.

But, actually, compiling uprobe_multi binary with -O2 takes a really
long time, and is quite useless (it's not a benchmark). So in addition
to ensuring that uprobe_multi_func_XXX functions are preserved, opt-out
of -O2 explicitly in Makefile and stick to -O0. This saves a lot of
compilation time.

With -O2, just recompiling uprobe_multi:

  $ touch uprobe_multi.c
  $ time make RELEASE=1 -j90
  make RELEASE=1 -j90  291.66s user 2.54s system 99% cpu 4:55.52 total

With -O0:
  $ touch uprobe_multi.c
  $ time make RELEASE=1 -j90
  make RELEASE=1 -j90  22.40s user 1.91s system 99% cpu 24.355 total

5 minutes vs (still slow, but...) 24 seconds.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240329190410.4191353-1-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/Makefile

@@ -759,7 +759,7 @@ $(OUTPUT)/veristat: $(OUTPUT)/veristat.o
 
 $(OUTPUT)/uprobe_multi: uprobe_multi.c
 	$(call msg,BINARY,,$@)
-	$(Q)$(CC) $(CFLAGS) $(LDFLAGS) $^ $(LDLIBS) -o $@
+	$(Q)$(CC) $(CFLAGS) -O0 $(LDFLAGS) $^ $(LDLIBS) -o $@
 
 EXTRA_CLEAN := $(SCRATCH_DIR) $(HOST_SCRATCH_DIR)			\
 	prog_tests/tests.h map_tests/tests.h verifier/tests.h		\

tools/testing/selftests/bpf/uprobe_multi.c

@@ -9,7 +9,7 @@
 
 #define NAME(name, idx) PASTE(name, idx)
 
-#define DEF(name, idx)  int NAME(name, idx)(void) { return 0; }
+#define DEF(name, idx) int __attribute__((weak)) NAME(name, idx)(void) { return 0; }
 #define CALL(name, idx) NAME(name, idx)();
 
 #define F(body, name, idx) body(name, idx)