libdfp: Always link tests with libdfp

This patch adds -Wl,--no-as-needed linker option when building tests to
force linker always use libdfp, even for cases where tests do not
actually uses libdfp symbols (test-numdigits for instance).

This is required because some tests will try to printf decimal floating
point and it requires proper initialization done by libdfp.so constructor.
(specially when building with fortify options).