selftests/bpf: Build everything in debug mode

Build selftests, bpftool, and libbpf in debug mode with DWARF data to
facilitate easier debugging.

In terms of impact on building and running selftests. Build is actually faster
now:

BEFORE: make -j60  380.21s user 37.87s system 1466% cpu 28.503 total
AFTER:  make -j60  345.47s user 37.37s system 1599% cpu 23.939 total

test_progs runtime seems to be the same:

BEFORE:
real    1m5.139s
user    0m1.600s
sys     0m43.977s

AFTER:
real    1m3.799s
user    0m1.721s
sys     0m42.420s

Huge difference is being able to debug issues throughout test_progs, bpftool,
and libbpf without constantly updating 3 Makefiles by hand (including GDB
seeing the source code without any extra incantations).
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210313210920.1959628-5-andrii@kernel.org

tools/testing/selftests/bpf/Makefile

@@ -21,7 +21,7 @@ endif
 
 BPF_GCC		?= $(shell command -v bpf-gcc;)
 SAN_CFLAGS	?=
-CFLAGS += -g -rdynamic -Wall -O2 $(GENFLAGS) $(SAN_CFLAGS)		\
+CFLAGS += -g -Og -rdynamic -Wall $(GENFLAGS) $(SAN_CFLAGS)		\
 	  -I$(CURDIR) -I$(INCLUDE_DIR) -I$(GENDIR) -I$(LIBDIR)		\
 	  -I$(TOOLSINCDIR) -I$(APIDIR) -I$(OUTPUT)			\
 	  -Dbpf_prog_load=bpf_prog_test_load				\
@@ -201,6 +201,7 @@ $(DEFAULT_BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile)    \
 		    $(HOST_BPFOBJ) | $(HOST_BUILD_DIR)/bpftool
 	$(Q)$(MAKE) $(submake_extras)  -C $(BPFTOOLDIR)			       \
 		    CC=$(HOSTCC) LD=$(HOSTLD)				       \
+		    EXTRA_CFLAGS='-g -Og'				       \
 		    OUTPUT=$(HOST_BUILD_DIR)/bpftool/			       \
 		    prefix= DESTDIR=$(HOST_SCRATCH_DIR)/ install
 
@@ -218,6 +219,7 @@ $(BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)		       \
 	   ../../../include/uapi/linux/bpf.h                                   \
 	   | $(INCLUDE_DIR) $(BUILD_DIR)/libbpf
 	$(Q)$(MAKE) $(submake_extras) -C $(BPFDIR) OUTPUT=$(BUILD_DIR)/libbpf/ \
+		    EXTRA_CFLAGS='-g -Og'					       \
 		    DESTDIR=$(SCRATCH_DIR) prefix= all install_headers
 
 ifneq ($(BPFOBJ),$(HOST_BPFOBJ))
@@ -225,6 +227,7 @@ $(HOST_BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)                \
 	   ../../../include/uapi/linux/bpf.h                                   \
 	   | $(INCLUDE_DIR) $(HOST_BUILD_DIR)/libbpf
 	$(Q)$(MAKE) $(submake_extras) -C $(BPFDIR)                             \
+		    EXTRA_CFLAGS='-g -Og'					       \
 		    OUTPUT=$(HOST_BUILD_DIR)/libbpf/ CC=$(HOSTCC) LD=$(HOSTLD) \
 		    DESTDIR=$(HOST_SCRATCH_DIR)/ prefix= all install_headers
 endif