[lldb] [test] Do not link -ldl on NetBSD

Fix the load_* using test Makefiles not to link -ldl on NetBSD.
There is no such a library on NetBSD, and dlopen() is available
without a library.  Quoting the manpage:

    (These functions are not in a library.  They are included in every
    dynamically linked program automatically.)

To resolve this portably, introduce a new USE_LIBDL option.  If it set
to 1, Makefile.rules automatically appends -ldl on platforms needing it.

Differential Revision: https://reviews.llvm.org/D58517

llvm-svn: 354617