selftests/bpf: fix building bpf selftests statically

bpf selftests can no longer be built with CFLAGS=-static with
liburandom_read.so and its dependent target.

Filter out -static for liburandom_read.so and its dependent target.

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>

Author: yosrym93

tools/testing/selftests/bpf/Makefile

@@ -168,14 +168,17 @@ $(OUTPUT)/%:%.c
 	$(call msg,BINARY,,$@)
 	$(Q)$(LINK.c) $^ $(LDLIBS) -o $@
 
+# If the tests are being built statically, exclude dynamic libraries defined
+# in this Makefile and their dependencies.
+DYNAMIC_CFLAGS := $(filter-out -static,$(CFLAGS))
 $(OUTPUT)/liburandom_read.so: urandom_read_lib1.c urandom_read_lib2.c
 	$(call msg,LIB,,$@)
-	$(Q)$(CC) $(CFLAGS) -fPIC $(LDFLAGS) $^ $(LDLIBS) --shared -o $@
+	$(Q)$(CC) $(DYNAMIC_CFLAGS) -fPIC $(LDFLAGS) $^ $(LDLIBS) --shared -o $@
 
 $(OUTPUT)/urandom_read: urandom_read.c urandom_read_aux.c $(OUTPUT)/liburandom_read.so
 	$(call msg,BINARY,,$@)
-	$(Q)$(CC) $(CFLAGS) $(LDFLAGS) $(filter %.c,$^)			       \
-		  liburandom_read.so $(LDLIBS)	       			       \
+	$(Q)$(CC) $(DYNAMIC_CFLAGS) $(LDFLAGS) $(filter %.c,$^)			\
+		  liburandom_read.so $(LDLIBS)					\
 		  -Wl,-rpath=. -Wl,--build-id=sha1 -o $@
 
 $(OUTPUT)/bpf_testmod.ko: $(VMLINUX_BTF) $(wildcard bpf_testmod/Makefile bpf_testmod/*.[ch])