selftests/bpf: Prevent skeleton generation race

Prevent "classic" and light skeleton generation rules from stomping on
each other's toes due to the use of the same <obj>.linked{1,2,3}.o
naming pattern. There is no coordination and synchronizataion between
.skel.h and .lskel.h rules, so they can easily overwrite each other's
intermediate object files, leading to errors like:

  /bin/sh: line 1: 170928 Bus error               (core dumped)
  /data/users/andriin/linux/tools/testing/selftests/bpf/tools/sbin/bpftool gen skeleton
  /data/users/andriin/linux/tools/testing/selftests/bpf/test_ksyms_weak.linked3.o
  name test_ksyms_weak
  > /data/users/andriin/linux/tools/testing/selftests/bpf/test_ksyms_weak.skel.h
  make: *** [Makefile:507: /data/users/andriin/linux/tools/testing/selftests/bpf/test_ksyms_weak.skel.h] Error 135
  make: *** Deleting file '/data/users/andriin/linux/tools/testing/selftests/bpf/test_ksyms_weak.skel.h'

Fix by using different suffix for light skeleton rule.

Fixes: c48e51c8 ("bpf: selftests: Add selftests for module kfunc support")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220509004148.1801791-2-andrii@kernel.org