selftests: Fix the .c linking rule

selftests: Fix the .c linking rule

Currently we can't build some tests, for example:

  $ make -C tools/testing/selftests/ TARGETS=vm
  ...
  gcc -Wall -I ../../../../usr/include   -lrt -lpthread ../../../../usr/include/linux/kernel.h userfaultfd.c -o tools/testing/selftests/vm/userfaultfd
  /tmp/ccmOkQSM.o: In function `stress':
  userfaultfd.c:(.text+0xc60): undefined reference to `pthread_create'
  userfaultfd.c:(.text+0xca5): undefined reference to `pthread_create'
  userfaultfd.c:(.text+0xcee): undefined reference to `pthread_create'
  userfaultfd.c:(.text+0xd30): undefined reference to `pthread_create'
  userfaultfd.c:(.text+0xd77): undefined reference to `pthread_join'
  userfaultfd.c:(.text+0xe7d): undefined reference to `pthread_join'
  userfaultfd.c:(.text+0xe9f): undefined reference to `pthread_cancel'
  userfaultfd.c:(.text+0xec6): undefined reference to `pthread_join'
  userfaultfd.c:(.text+0xf14): undefined reference to `pthread_join'
  /tmp/ccmOkQSM.o: In function `userfaultfd_stress':
  userfaultfd.c:(.text+0x13e2): undefined reference to `pthread_attr_setstacksize'
  collect2: error: ld returned 1 exit status

This is because the rule for linking .c files to binaries is incorrect.

The first bug is that it uses $< (first prerequisite) instead of $^ (all
preqrequisites), fix it by using ^$.

Secondly the ordering of the prerequisites vs $(LDLIBS) is wrong,
meaning on toolchains that use --as-needed we fail to link (as above).
Fix that by placing $(LDLIBS) *after* ^$.

Finally switch to using the default rule $(LINK.c), so that we get
$(CPPFLAGS) etc. included.

Fixes: a8ba798bc8ec ("selftests: enable O and KBUILD_OUTPUT")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>