benchtests: Create objdir in the bench-%.c generation rule

benchtests: Create objdir in the bench-%.c generation rule

The $(objpfx)bench-%.c rule writes its output into $(objpfx) without
ensuring that directory exists.  Serial builds happened to satisfy
that ordering, with parallel builds the generation recipe can
run before the directory is created, failing with:

  cannot create .../benchtests/bench-xxx.c-tmp: Directory nonexistent

Add the standard $(make-target-directory).

Reviewed-by: Florian Weimer <fweimer@redhat.com>