selftests: use LDLIBS for libraries instead of LDFLAGS

While building selftests, the following errors were observed:
> tools/testing/selftests/timens'
> gcc -Wall -Werror -pthread  -lrt -ldl  timens.c  -o tools/testing/selftests/timens/timens
> /usr/bin/ld: /tmp/ccGy5CST.o: in function `check_config_posix_timers':
> timens.c:(.text+0x65a): undefined reference to `timer_create'
> collect2: error: ld returned 1 exit status

Quoting commit 870f193d ("selftests: net: use LDLIBS instead of
LDFLAGS"):

The default Makefile rule looks like:

$(CC) $(CFLAGS) $(LDFLAGS) $@ $^ $(LDLIBS)

When linking is done by gcc itself, no issue, but when it needs to be passed
to proper ld, only LDLIBS follows and then ld cannot know what libs to link
with.

More detail:
https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html

LDFLAGS
Extra flags to give to compilers when they are supposed to invoke the linker,
‘ld’, such as -L. Libraries (-lfoo) should be added to the LDLIBS variable
instead.

LDLIBS
Library flags or names given to compilers when they are supposed to invoke the
linker, ‘ld’. LOADLIBES is a deprecated (but still supported) alternative to
LDLIBS. Non-library linker flags, such as -L, should go in the LDFLAGS
variable.

While at here, correct other selftests, not only timens ones.
Reported-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Tested-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/futex/functional/Makefile

 # SPDX-License-Identifier: GPL-2.0
 INCLUDES := -I../include -I../../
 CFLAGS := $(CFLAGS) -g -O2 -Wall -D_GNU_SOURCE -pthread $(INCLUDES)
-LDFLAGS := $(LDFLAGS) -pthread -lrt
+LDLIBS := -lpthread -lrt
 
 HEADERS := \
 	../include/futextest.h \

tools/testing/selftests/net/Makefile

@@ -27,5 +27,5 @@ KSFT_KHDR_INSTALL := 1
 include ../lib.mk
 
 $(OUTPUT)/reuseport_bpf_numa: LDLIBS += -lnuma
-$(OUTPUT)/tcp_mmap: LDFLAGS += -lpthread
-$(OUTPUT)/tcp_inq: LDFLAGS += -lpthread
+$(OUTPUT)/tcp_mmap: LDLIBS += -lpthread
+$(OUTPUT)/tcp_inq: LDLIBS += -lpthread

tools/testing/selftests/rtc/Makefile

 # SPDX-License-Identifier: GPL-2.0
 CFLAGS += -O3 -Wl,-no-as-needed -Wall
-LDFLAGS += -lrt -lpthread -lm
+LDLIBS += -lrt -lpthread -lm
 
 TEST_GEN_PROGS = rtctest
 

tools/testing/selftests/timens/Makefile

@@ -2,6 +2,6 @@ TEST_GEN_PROGS := timens timerfd timer clock_nanosleep procfs exec
 TEST_GEN_PROGS_EXTENDED := gettime_perf
 
 CFLAGS := -Wall -Werror -pthread
-LDFLAGS := -lrt -ldl
+LDLIBS := -lrt -ldl
 
 include ../lib.mk