selftests: remove CROSS_COMPILE in dedicated Makefile

After previous clean up patches, memfd and timers could get
CROSS_COMPILE from tools/testing/selftest/lib.mk. There is no need to
preserve these definition. So, this patch remove them.
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/memfd/Makefile

-CC = $(CROSS_COMPILE)gcc
 CFLAGS += -D_FILE_OFFSET_BITS=64
 CFLAGS += -I../../../../include/uapi/
 CFLAGS += -I../../../../include/

tools/testing/selftests/timers/Makefile

-CC = $(CROSS_COMPILE)gcc
 BUILD_FLAGS = -DKTEST
 CFLAGS += -O3 -Wl,-no-as-needed -Wall $(BUILD_FLAGS)
 LDFLAGS += -lrt -lpthread