testsuite: Fix build when linking with lld

testsuite: Fix build when linking with lld

When building with clang and linking with lld there is an issue with
keeping the kmod_tests sections: On ELF targets, __attribute__((used))
prevents compiler discarding, but does not affect linker --gc-sections,
according to https://lld.llvm.org/ELF/start-stop-gc.

Make sure the _start/_stop symbols are not weak and add the "retain"
attribute.

Closes: https://github.com/kmod-project/kmod/issues/269
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/274