selftests/bpf: Fix vmtest static compilation error

As stated in README.rst, in order to resolve errors with linker errors,
'LDLIBS=-static' should be used. Most problems will be solved by this
option, but in the case of urandom_read, this won't fix the problem. So
the Makefile is currently implemented to strip the 'static' option when
compiling the urandom_read. However, stripping this static option isn't
configured properly on $(LDLIBS) correctly, which is now causing errors
on static compilation.

    # LDLIBS=-static ./vmtest.sh
    ld.lld: error: attempted static link of dynamic object liburandom_read.so
    clang: error: linker command failed with exit code 1 (use -v to see invocation)
    make: *** [Makefile:190: /linux/tools/testing/selftests/bpf/urandom_read] Error 1
    make: *** Waiting for unfinished jobs....

This commit fixes this problem by configuring the strip with $(LDLIBS).

Fixes: 68084a13 ("selftests/bpf: Fix building bpf selftests statically")
Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230125100440.21734-1-danieltimlee@gmail.com

tools/testing/selftests/bpf/Makefile

@@ -181,14 +181,15 @@ endif
 # do not fail. Static builds leave urandom_read relying on system-wide shared libraries.
 $(OUTPUT)/liburandom_read.so: urandom_read_lib1.c urandom_read_lib2.c
 	$(call msg,LIB,,$@)
-	$(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $^ $(LDLIBS)   \
+	$(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS))   \
+		     $^ $(filter-out -static,$(LDLIBS))	     \
 		     -fuse-ld=$(LLD) -Wl,-znoseparate-code -Wl,--build-id=sha1 \
 		     -fPIC -shared -o $@
 
 $(OUTPUT)/urandom_read: urandom_read.c urandom_read_aux.c $(OUTPUT)/liburandom_read.so
 	$(call msg,BINARY,,$@)
 	$(Q)$(CLANG) $(filter-out -static,$(CFLAGS) $(LDFLAGS)) $(filter %.c,$^) \
-		     liburandom_read.so $(LDLIBS)			       \
+		     liburandom_read.so $(filter-out -static,$(LDLIBS))	     \
 		     -fuse-ld=$(LLD) -Wl,-znoseparate-code -Wl,--build-id=sha1 \
 		     -Wl,-rpath=. -o $@